It's the first button, right before the paragraph mark (Showing visible characters).
This is the first in a series, which one might call:
What does FEATURE X do?
Figured it out:
Record/List selected words =
Toggle word recording. While recording, pressing F5 will save the selected word to a list for later reference. Pressing F5 with your selection will list the saved words (with their contexts).
I added it to the wiki, @ http://cafetran.wikidot.com/the-edit-menu