The TagDefinition holds the definition of a given type of tagged value: where it is to be used, what values it typically holds or must hold, what data type these values are, and so on.

The type of a tagged value object is given by its name or name stem. A name stem is the prefix in a delimited name, where the default delimiter is "." For example, consider the following tagged value tags:

Tag definitions are not automatically enforced, except in the Workbench tagged values editor (Extension Properties dialog). Instead, they provide a means, available to developers at runtime, to characterize or validate tags that have characteristic behavior.

For example, tags of a particular type may have a fixed set of possible values; creating a tag definition for such a tag type enables the appropriate value set to be collated and fetched at runtime and used for lookup or validation. Similarly the "subtype" tag type, used by the Workbench tagged values editor when editing attributes, enables the appropriate formatting to be identified and applied to fields displaying a given attribute.

For examples of how tag definitions are used, examine the Workbench tagged values editor for user classes and attributes, and the tag definitions used by the editor (in the file %II_SYSTEM%\files\appedtags.xml).