I know how AP behaves today if you only engage one or the other. For example, if you only engage TACC it will manage the vehicle's speed but will not steer. If you do the opposite it will steer but you have to manage speed.
How does it work with FSD? If you engage TACC only does it simply maintain speed? If you engage auto steer only does it only lane keep, or will it still make turns at intersections?
You can't enable just Autosteer. TACC is required, both now in the public builds and in the FSD builds.