Help Current
Help->Current describes the task currently active.

Pressing Atom->Create and then Help->Current on the top menu, a window Help Atom Create pops up showing how to use the Atom Create dialog. Pressing then Cell->Modify and Help->Current, a window Help Cell Modify replaces the previous help window, showing how to handle the Cell Modify dialog.

Help->Current is conveniently set up as the first option in the Help menu.