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A feature system declaration (FSD) is an auxiliary file used in conjunction with a certain type of text that makes use of fs (that is, feature structure) elements. The FSD serves four purposes. 1) It provides an encoding by which types and their subtyping and inheritance relationships can be introduced and defined, thus laying the basis for constructing a feature system. 2) It provides a mechanism by which the encoder can list all of the feature names and feature values and give a prose description as to what each represents. 3) It provides a mechanism by which type constraints can be declared, against which typed feature structures are validated relative to a given theory stated in typed feature logic. These constraints may involve constraints on the range of a feature's value, constraints on which features are permitted within certain types of feature structures, or constraints that prevent the co-occurrence of certain feature-value pairs. The source of these constraints is normally the empirical domain being modelled. 4) It provides a mechanism by which the encoder can define the intended interpretation of underspecified feature structures. This involves defining default values (whether literal or computed) for missing features.
The scheme described in ISO 24610-2:2011 may be used to document any feature system, but is primarily intended for use with the typed feature structure representation defined in ISO 24610-1. The feature structure representations of ISO 24610-1 specify data structures that are subject to the typing conventions and constraints specified using ISO 24610-2:2011. The feature structure representations of ISO 24610-1 are also used within some of the elements defined in ISO 24610-2:2011.