Non-RDB file

A customer needs us to send them our invoicing data in a format that is non-compliant with programming standards I have used for past 20+ years where all records in a file are the same layout.
Instead, if first field of record is
* Type-A header content then rest of record is type-A layout with like name & address info that's going to be pretty consistent since bill-to never changes, ship-to rarely
* Type-B line content then rest of record is type-B layout with details on what we shipped
* Type-C totals content then rest of record is type-C layout with kind of info we find at foot of any invoice
The resulting file will be like ABBBCABCABBCABBBBBC with any number of B records in each set of 3 kinds

I need a mind-refresher of terminology where to look in DDS+etc. manuals, how to set this up & process it.

I'm thinking maybe multi-format file.
Any program to create / access the file would need to either
* define it 3 times, one per format type, using matching records on invoice # to control input sequencing, in the old RPG cycle,
* or read input as a "flat file" examine first field, then dump rest of each record into A B or C corresponding layout data structure

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Al Macintyre
Jack of many 400 trades, master of some, but not enough