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A.6 Formal System Identifier Definition
Requirements (FSIDR)
An entity is a virtual storage object. A system identifier parameter of a markup declaration can be used to map an entity onto one or more real storage objects (or portions thereof), and to specify processes to be performed in the course of accessing the object as an entity. The format of a system identifier is normally system-specific (an "informal system identifier"). However, when access to storage is specified in the manner described in this annex, the system identifier is called a "formal system identifier" (FSI).
NOTE 497 A given system identifier is either informal or formal. Both kinds can occur in the same document.
A formal system identifier consists of one or more "storage object specifications (SOS)", each of which identifies a storage object. The storage objects are concatenated in the order specified to comprise the storage of the entity. When the "altsos" option is supported, the FSI can contain alternative SOS sequences that identify alternative (that is, duplicate) storage locations for the identical objects; the system is free to choose the most convenient.
The format of an SOS resembles an element, in that it consists of a start-tag followed by content, followed optionally by an end-tag or empty end-tag.
In the SOS tag, the name appearing as the generic identifier is that of the storage manager (SMName). It is the name of a "storage manager notation" that was identified as such by a declaration. There can also be an attribute specification list, consisting of attributes defined for the storage manager notation. These SOS attributes serve as parameters that govern the access to the storage object.
The content of an SOS is known as the "storage object identifier" (SOI). Its syntax and semantics depend on the individual storage manager.
A system identifier is recognized as an FSI only if it begins with an SOS tag. Informal system identifiers can be used in the same document as FSIs as long as storage manager names are chosen so that informal system identifiers don't appear to begin with an SOS tag.
A system FSI definition document (fsidd) should define how informal system identifiers are interpreted. Typically, they are treated as SOIs of a particular SM, and given the default values of the SM's attributes.
An entity can have attributes provided by the notation in which the entity body is represented. These attributes are called "data attributes"; they describe aspects of interpreting the notation.
The storage objects in which an entity is stored can also have attributes. They are provided by the storage managers and describe aspects of physically accessing the objects and converting them to the form expected by the SGML parser or other notation interpreter.
An entity can have additional properties of its own that are independent of both the notation and the storage manager. They are called "entity usage attributes" and they describe aspects of the use of the entity, such as authorizations, modification levels, and alternative representations.
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