You should also refer your vendor to 2.6.2 of the standard, which reads:
2.6.2 Rules for the recipient
The following rules apply to receiving HL7 messages and converting their contents to data values:
a) ignore segments, fields, components, subcomponents, and extra repetitions of a field that are present but were not expected.
b) treat segments that were expected but are not present as consisting entirely of fields that are not present.
c) treat fields and components that are expected but were not included in a segment as not present
