The secondary-selection is used when the cursor is in some focussed Recipient window, in order to grab some text (or whatever) from some Donor window (possibly the same window as the Recipient) and have that text pasted at the Recipient’s insertion-point (overwriting any primary-selection in the Recipient).


There are lots of ways to move data between or within graphical windows! But the list is shrinking
https://en.wikipedia.org/wiki/X_Window_System_selection