I think I figured out what's written in plain English right in my face:

STR ArrayVar\n{\c}
Receive string of n characters optionally ended in character c

It looks like I can:

- define the array at the longest possible length,
- stop reception on character c,

This way I can embed a Data Layout Number at the start of the message, and use that to branch to code to treat the array properly.

I understand quickly, you just have to be patient and explain it to me a whole bunch of times.