what is the option :joinable in transactions

hi,
what does the option key :joinable do in
active_record/connection_adapters/abstract/database_statements.rb.

cheers,
deepak

It seems to be a way to say that the next nested call, if any,
shouldn't join this particular transaction. :requires_new takes effect
in the very call, :joinable saves state for an hypothetical next call.
I could be wrong and don't have a use case in mind, but it is what I
interpret after a quick read.

It is certainly undocummented.