The selection is best thought of as a cursor, although it looks rather like a GNUemacs transient region. It is normally displayed by setting the background colour of the text it contains (you can change how it is displayed: see Selection attribute). Versor editing operations act on the selection; for example, the versor “Delete” command deletes the current selection.
The selection covers one or more units of the current dimension (see Coordinates and dimensions). For how to make it cover more than one unit, see Extending.
The selection is distinct from the GNUemacs region, to avoid putting lots of marks into the mark ring.
The selection highlighting disappears when you do a non-versor action, and reappears when you do a versor action.