// 来自 vim help( :help x11-selection)
X11 provides two basic types of global store, selections and cut-buffers,
which differ in one important aspect: selections are "owned" by an
application, and disappear when that application (e.g., Vim) exits, thus
losing the data, whereas cut-buffers, are stored within the X-server itself
and remain until written over or the X-server exits (e.g., upon logging out).
The contents of selections are held by the originating application (e.g., upon
a copy), and only passed on to another application when that other application
asks for them (e.g., upon a paste).
The contents of cut-buffers are immediately written to, and are then
accessible directly from the X-server, without contacting the originating
application.
// :help quoteplus
There are three documented X selections: PRIMARY (which is expected to
represent the current visual selection - as in Vim's Visual mode), SECONDARY
(which is ill-defined) and CLIPBOARD (which is expected to be used for cut,
copy and paste operations).
Of these three, Vim uses PRIMARY when reading and writing the "* register
(hence when the X11 selections are available, Vim sets a default value for
|'clipboard'| of "autoselect"), and CLIPBOARD when reading and writing the "+
register. Vim does not access the SECONDARY selection.