You may freely add or remove items in your clipboard. To add a new term, click on the folder icon next to a keyword from the list. It will turn to check mark when it's done.
You may find it useful to analyse a few keywords at once. You can start analysis with the "Research" button. Then, "SERP similarity" turns to "max SERP similarity". Hover the max SERP similarity value to get precise info on similarity between a particular term.