Although you are free to define your own metadata, by using any key (according to the syntax), some keys have a special meaning that is enforced by Zettelstore. See the computed list of supported metadata keys for details.
Most keys conform to a type.
author
A string value describing the author of a zettel. If given, it will be shown in the web user interface for the zettel.
back
Is a property that contains the identifier of all zettel that reference the zettel of this metadata, that are not referenced by this zettel. Basically, it is the value of backward, but without any zettel identifier that is contained in forward.
backward
Is a property that contains the identifier of all zettel that reference the zettel of this metadata. References within invertible values are not included here, e.g. precursor.
box-number
Is a computed value and contains the number of the box where the zettel was found. For all but the predefined zettel, this number is equal to the number X specified in startup configuration key box-uri-__X__.
copyright
Defines a copyright string that will be encoded. If not given, the value default-copyright from the configuration zettel will be used.
created
Date and time when a zettel was created through Zettelstore. If you create a zettel with an editor software outside Zettelstore, you should set it manually to an appropriate value.
This is a computed value. There is no need to set it via Zettelstore.  If it is not stored within a zettel, it will be computed based on the value of the Zettel Identifier: if it contains a value >= 19700101000000, it will be coerced to da date/time; otherwise the version time of the running software will be used.
Please note that the value von created will be different (in most cases) to the value of id / the zettel identifier, because it is exact up to the second. When calculating a zettel identifier, Zettelstore tries to set the second value to zero, if possible.
created-missing
If set to true, the value of created was not stored within a zettel. To allow the migration of zettel identifier to a new scheme, you should update the value of created to a reasonable value. Otherwise you might lose that information in future releases.
This key will be removed when the migration to a new zettel identifier format has been completed.
credential
Contains the hashed password, as it was emitted by zettelstore password. It is internally created by hashing the password, the zettel identifier, and the value of the ident key.
It is only used for zettel with a role value of user.
dead
Property that contains all references that does not identify a zettel.
expire
A user-entered time stamp that document the point in time when the zettel should expire. When a zettel is expires, Zettelstore does nothing. It is up to you to define required actions. expire is just a documentation. You could define a query and execute it regularly, for example expire? ORDER expire. Alternatively, a Zettelstore client software could define some actions when it detects expired zettel.
folge
Is a property that contains identifier of all zettel that reference this zettel through the precursor value.
folge-role
Specifies a suggested role the zettel should use in the future, if zettel currently has a preliminary role.
forward
Property that contains all references that identify another zettel within the content of the zettel.
id
Contains the zettel identifier, as given by the Zettelstore. It cannot be set manually, because it is a computed value.
lang
Language for the zettel. Mostly used for HTML rendering of the zettel.
If not given, the value lang from the zettel of the current user will be used. If that value is also not available, it is read from the configuration zettel will be used. Use values according to the language definition of RFC-5646.
license
Defines a license string that will be rendered. If not given, the value default-license from the configuration zettel will be used.
modified
Date and time when a zettel was modified through Zettelstore. If you edit a zettel with an editor software outside Zettelstore, you should set it manually to an appropriate value.
This is a computed value. There is no need to set it via Zettelstore.
precursor
References zettel for which this zettel is a Folgezettel / follow-up zettel. Basically the inverse of key folge.
predecessor
References the zettel that contains a previous version of the content. In contrast to precursor / folge, this is a reference because of technical reasons, not because of content-related reasons. Basically the inverse of key successors.
prequel
Specifies a zettel that is conceptually a prequel zettel. This is a zettel that occured somehow before the current zettel.
published
This property contains the timestamp of the mast modification / creation of the zettel. If modified is set with a valid timestamp, it contains the its value. Otherwise, if created is set with a valid timestamp, it contains the its value. Otherwise, if the zettel identifier contains a valid timestamp, the identifier is used. In all other cases, this property is not set.
It can be used for sorting zettel based on their publication date.
It is a computed value. There is no need to set it via Zettelstore.
query
Stores the query that was used to create the zettel. This is for future reference.
read-only
Marks a zettel as read-only. The interpretation of supported values for this key depends, whether authentication is enabled or not.
role
Defines the role of the zettel. Can be used for selecting zettel. See supported zettel roles. If not given, it is ignored.
sequel
Is a property that contains identifier of all zettel that reference this zettel through the prequel value.
successors
Is a property that contains identifier of all zettel that reference this zettel through the predecessor value. Therefore, it references all zettel that contain a new version of the content and/or metadata. In contrast to folge, these are references because of technical reasons, not because of content-related reasons. In most cases, zettel referencing the current zettel should be updated to reference a successor zettel. The query reference backward? successors? lists all such zettel.
summary
Summarizes the content of the zettel. You may use all inline-structued elements of Zettelmarkup.
syntax
Specifies the syntax that should be used for interpreting the zettel. The zettel about other markup languages defines supported values. If it is not given, it defaults to plain.
tags
Contains a space separated list of tags to describe the zettel further. Each Tag must begin with the number sign character (#, U+0023).
title
Specifies the title of the zettel. If not given, the value of id will be used.
url
Defines an URL / URI for this zettel that possibly references external material. One use case is to specify the document that the current zettel comments on. The URL will be rendered special in the web user interface if you use the default template.
useless-files
Contains the file names that are rejected to serve the content of a zettel. Is used for directory boxes and file boxes. If a zettel is deleted, these files will also be deleted.
user-id
Provides some unique user identification for an user zettel. It is used as a user name for authentication.
It is only used for zettel with a role value of user.
user-role
Defines the basic privileges of an authenticated user, e.g. reading / changing zettel. Is only valid in a user zettel.
See User roles for more details.
visibility
When you work with authentication, you can give every zettel a value to decide, who can see the zettel. Its default value can be set with default-visibility of the configuration zettel.
See visibility rules for zettel for more details.