If you're trading SPX options, you sometimes need to distinguish between the AM expiration and the PM expiration. In thinkorswim, there are three ways to identify which is which.
Look at the following image, where we have two different rows in the option chain for SPX options expiring 17 Sep 2021. The first row shows a small "AM" symbol to the right of the date. That means that row is for the SPX options that expire at the open on Friday, which is when they settle. The next row, which doesn't have an AM symbol, represents the options that expire that day at the close of the trading day - the PM options.
You can also see that there's a difference in the number in parentheses that follows the date and time indications, which represents days until expiration. This image was taken on a Thursday, and the 17 SEP 2021 AM expiration shows 0 trading days left. The row that expires on Friday still has 1 full day left. The row with a full day of trading remaining would be the afternoon option.
The third way to identify them is to know that we generally want the Weekly options rather than the Monthly options. The weekly options are easily identified in the image, below.
If you're looking for the weekly options in TS, check this kb article.