Erik Kostson
Ansys Employee

I think this option that you probably refer to is in WB transient mode superposition only, and when using acceleration and base excitation option (see image below).

Only then, and when the transient is solved with mode super-position (not full), it becomes possible to set this option.
(In a future release, e.g., 2023 R1, it might become possible to use base excitation in full transient, but it is not possible now)
Thank you