Use for the "Active" timestamp displayed above questions; for the "Last seen" on a user profile, use [profile-page]
The last activity of a question is when it was last touched as a whole for its entire being. That includes its answers, but not any comments on such.
When viewing question lists, you can click on the mentioned modified time to jump directly to the edit or answer that caused the question move or shift about.
modified Sep 10 at 15:14
In the above example display, if you clicked the time it will take you to the last made edit to the question or if it was done on an answer, to that.