The mouse can be put into three kinds of modes The one you clicked now puts it into a selection mode In this mode, the mouse cursor turns into the regular mouse cursor you see normally in Windows i.e. an arrow pointing slightly to left.

When the mouse is in this mode, then you can click and drag to mark out a rectangle on the site and TAD examines what was inside that marked rectangle. If there are corners of some objects in there; then TAD would select those objects. If those objects were already selected; then TAD would de-select them. Read more on selection with mouse

If there were absolutely NO corner in that marked out rectangle; then TAD would change the step size of the architect. That means the size of the architect would change as per that settings

Read more on the step size