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There are two basic vocabularies of data dictionary. The first is the business object data dictionary, which provides metadata about KFS business objects: what their inquiry pages look like, how their lookups work, relationships with external business objects and details about it's fields like size, label, display control, field-security etc.. There's a full explanation of the business object data dictionary here.
Then there are document data dictionary entries, which are split into two similar but distinct vocabularies: the maintenance document data dictionary and the transactional document data dictionary. A fuller discussion of those can be found here. In each of those data dictionaries, we can set authorizations, rules, workflow searchable attributes, workflow route node qualifiers, and other useful stuff.
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