Hi Prathibha,
Two ways, i see, that this can be done.
Create Data Entry Profiles based on the A/Atypes... and in the default values select both Activity Type + A/A Type and input the A/A type in the field available. could get confusing after some point.
Second way is to implement the required change in User Exit CATS0002.. Recommended way.
(Tcode SMOD/Enhancement CATS0002).
Hope this helps.
Kumarpal
