Thanks,

… I used UC hierarchy with auto-level numbering…. to get something like this:

UC1 – Base UseCase
UC1.1 – Derived UseCase 1
UC1.2 – Derived UseCase 2

… in RTF gerenation I user this level to put IDs to them…

… I suppose there is no option to do this in eaDocx… (Because there is no way to put some static string before prefix)…
(I am not asking this as a feature… EA should have usable Auto name counters… but it haven’t )