CAP694 states the following:
NUMBER OF AIRCRAFT (2 characters)
INSERT the number of aircraft, only if more than one (e.g. 03).
c) TYPE OF AIRCRAFT (2 to 4 characters)
INSERT the appropriate Aircraft Type Designator as specified in ICAO DOC 8643
Aircraft Type Designators;
OR
if no such designator has been assigned or in case of formation flights comprising
more than one type, INSERT "ZZZZ" and specify in Item 18 the numbers and type(s)
of aircraft preceded by TYP/.
Not much more guidance it seems, like whether to include registrations of the other aircraft, how to complete the supplementary information etc.