Merging two 3.8 boards, which is source?

[jwocky]

Sorry, Im new to IMPEX system. I have one very large forum (forum A) and one very small forum (forum B).

I want to merge forum B into forum A, so that all the users and threadid's for forum A are intact and unchanged, and the threadids for forum B can be changed as part of the merge into forum A.

Im confused as to which board is then the source board etc?

Thanks so much for help!