ISO 10303-21:2016(E)
The exchange structure is described by an unambiguous, context-free grammar to facilitate parsing by software. The grammar is expressed in Wirth Syntax Notation that is described in annex B. The form of product data in the exchange structure is specified using a mapping from the EXPRESS language to the exchange structure syntax.
Any quotation marks used in this part of ISO 10303 are not part of the text that appears in the exchange structure but serve to delimit that text. This statement applies to all places in the text where quotation marks are used. Table 2, Table 3, and Table 4 form an exception to this rule as the quotation marks used in those tables form part of the WSN rules.
In ISO/IEC 8859, each character is assigned an identifying name. When that name is used in this part of ISO 10303, it is typeset in italics to distinguish it from ordinary text. Thus comma is used to refer to ",", low line refers to "_", and latin capital letter A refers to "A".
Within examples in this part of ISO 10303, an annotation is introduced by the sequence ----> where clarification is required.
Two levels of conformance are specified:
NOTE 1 Annex E presents methods for evaluating schema conformance when an exchange structure contains multiple data sections based on different EXPRESS schemas.
Syntactical conformance is a prerequisite for schema conformance.
Three classes of syntactical conformance are defined by this part of ISO 10303:
NOTE 2 A processor for a previous edition of this part of ISO 10303 can parse syntactical conformance class 1 by ignoring the anchor and signature sections, and it can parse syntactical conformance class 2 by using a pre-processor to resolve external entity references in the reference section.
NOTE 3 The syntactical conformance class of an exchange structure is shown by the implementation_level attribute of the file_description entity in the header section (see 8.2.2). If the value of this attribute is "4;1" then this exchange structure has syntactical conformance class 1. If the value of this attribute is "4;2" then this exchange structure has syntactical conformance class 2. If the value of this attribute is "4;3" then this exchange structure has syntactical conformance class 3.
An implementation that claims schema conformance to this part of ISO 10303 shall read or write files or both that exhibit schema as well as syntactical conformance.
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