cc @Martín Dutra this is the thread you should reference. I'll weave this into the ssb-post spec I'm writing for a complete picture this week

Thanks. This is really informative, I’m glad it will be a SIP of its own.

Based on my tests using Patchwork and Manyverse, both clients support creating a sub-thread without setting a fork, that is they display it as a sub-thread just taking the root into account. So I guess it should be optional, as to give additional information to some clients, but not entirely needed.