No subject

The reference encoder uses a single block size for the whole stream
but the FLAC format does not require it.

Should stream MUST have constant (fixed) block size (if blocksize
satisfies all restrictions from first question) to be subset?

Is variable block size stream (if blocksize satisfies all restrictions
from first question) be subset?

The third question:

Is FLAC files compressed by FFMPEG with "-compression_level 12" switch
are subset?