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).
Wow that is ancient! Interresting feature, I wonder if it could be reimplemented as a wayland protocol, however I think some modern IDEs and some text editors have something similar nowadays.