Thank you for subscribing to Annotated Thomist...check back each day for a new section of St. Thomas' corpus, annotated and summarized. (FREE TRIAL FOR NEW SUBSCRIBERS!!!)
AT is also available to donors of $10 or more on Patreon or SubscribeStar along with all of the other benefits (daily bonus videos, bonus articles, PDFs, etc.)
If you need more personalized help reading the Summa, I am available for 1-on-1 sessions, here.
cf., Sent.I.D38.Q1.A4; Sent.III.D14.Q1.A2.qa2; SCG1.C66; QDeVer.Q2.A8
The question is asking whether God knows things that are not actually. St. Thomas answers in the affirmative since there are many other modes of existence than actual existence, such as potential existence. Thus, God knows all those things he could have decreed, along with those things that He did.