This drill reviews the omitted letter 'r' in outlines. With the Anniversary edition, they found that most people will not pronounce the letter 'r' in words, so there was no reason to write it. Later it was added back into editions because it did help with transcribing and the speed lost wasn't needed for office workers.
There are a lot of instruction on this letter and this post has a few. Please review all that you need to.
For a review, please see this post "confusion about "r" endings" HERE. It links to this post " Anni--The "R" " in the Q&A section that has a lot of questions and answers HERE.