hyperextensibility

back in the day(*) the UCL internet gang (Indra) built a IP over the international X.25 packet switched system, that operated by tunneling IP packets over virtual circuits (VCs) that the various  telcos operated.

to indicate that the VC was carrying IP rather than (say) remote  terminal traffic from the quaintly named PAD (Packet Assembler&Disassembler), they used a fiield in the VC setup, that could carry Call User  Data (arbitrary stuff up to 128 bytes as far as I recall) and this could even have a copy of the IP address and other useful metadata...

so this is the grrandparent of SMS which uses call user data in the old GSM call setup packet to carry text messages.

so SMS begat Whatsapp and Twitter and who knows what more universes of discourse.

So these "features" are a great deal more powerful than mere options.but also less dangerous than fully programmable protocols (like, say, SIP).

They are like "lifting" in programming languages. And as such should be celebrated.

I'm going to call protocols with such an apparently lowly, but actually grand affordance, hyperextensible, as they allow shortcuts to entire new worlds.

* bob braden, peter kirstein et al, probably around 1980.