Diff Bet F-02 and FB01

I am trying to post the journal entries but I am getting CONVT_NO_NUMBER runtime error using Call Transaction method.

I was looking at Session method as well, it calls FB01 transaction inspite of me giving 'F-02'. The screen fields are similar. Is there any difference between these two transactions on its own and wrt this posting.

Can you please help me in this. Your help will be appreciated...

With Regards

Leo