Definition:
(re)using redundant fields in an existing protocol stack beyond its initial intent in order to carry extra information so as to hide ours, if possible
e.g.
using fixed reserved fields (0x00, magic number, etc.)
could break conformity, as well as being pretty obvious
using optional extended fields
in conformity with the standards, though, doing so could introduce new characteristics into the traffic, making our "extra information" somehow noticeable
using discretionary fields (e.g. seq/window size in the TCP header)
in accordance with standards, without bringing in any recognisable characteristics(at least at the packet level), this way seems to be the most graceful one among others mentioned above