"X Selection" and VI Register

There are three documented X selections:

• PRIMARY (which is expected to represent the current visual selection); 
• SECONDARY (which is ill-defined);
• 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
• CLIPBOARD when reading and writing the "+ register.  
• Vim does not access the SECONDARY selection.