See T82309#1055564 T63675: Placing keyframe outside of NLA Strip frame range while in Tweak Mode no longer possible and their duplicates for how Default-off can cause confusion.

This is just for convenience since it allows animators to keyframe outside of the strip's bounds by default. This was likely off by default before since Syncing Length would undesirably shift the whole animation after leaving tweak mode (fixed by D7602: NLA: Fix Sync Length) and the animator wasn't able to keyframe outside the strip bounds anyways (fixed by D7533: NLA: Evaluate Tweak Strip Within Synced Action Bounds). Now it's better if the flag was on by default. While the animator is still roughly developing their animation NLA-wise, they won't have to worry about strip bound keying failures. When they are more certain of the strip bounds, they can disable the flag to prevent affecting the rest of the NLA system.