'a' Autoselect: If present, then whenever VISUAL mode is started,
or the Visual area extended, Vim tries to become the owner of
the windowing system's global selection. This means that the
Visually highlighted text is available for pasting into other
applications as well as into Vim itself. When the Visual mode
ends, possibly due to an operation on the text, or when an
application wants to paste the selection, the highlighted text
is automatically yanked into the "* selection register.
Thus the selection is still available for pasting into other
applications after the VISUAL mode has ended.
If not present, then Vim won't become the owner of the
windowing system's global selection unless explicitly told to
by a yank or delete operation for the "* register.
The same applies to the modeless selection.