That's a bit of a longer question, but I have some notes I can share from Fallout 3. It's not one flag, its many.
http://www.gamesas.com/index.php?/topic/981311-proper-use-of-havok-data/page__view__findpost__p__14239714 that talks about Havok data and what the flags mean, warning - it's quite a read!
When I change stuff, here is my abbreviated version:
Layer: OL_STATIC (statics) or OL_PROPS (physics)Layer Copy: OL_STATIC (statics) or OL_PROPS (physics)Motion System: MO_SYS_FIXED (statics) or MO_SYS_BOX (physics - there are other types of MO_SYS_**** as well, check the flags in NifSkope for a complete list)Solver Deactivation: SOLVER_DEACTIVATION_OFF (statics) or SOLVER_DEACTIVATION_LOW (physics - there are other options here too, refer to above post for further reading)Quality Type: MO_QUAL_FIXED (statics) or MO_QUAL_DEBRIS (physics)
Now there may be other settings that could be beneficial, and there are Alot of options in the flagging as you can see from the post, but the ones I quote above come from objects in Fallout 3/NV; The common ashtray provides the flagging for objects we want to have physics, and a common box/storage container provides the static flagging. Dozens and dozens of each type of object (static and with physics) use the above flagging. Still I encourage you to try them out and test for yourself.
One of the modders with more havok expertise may be able to provide us with further insight as well.
Otherwise, good luck!
Miax