Thanks for joining us here in the Community. I'd be glad to provide some info about the tagging feature in QuickBooks.
At this time, the ability to tag expenses and other transactions is only available in QuickBooks Online, and not the Self-Employed version. I recommend submitting feedback to the Product Development Team to let them know you'd like to see the option to use tagging in QuickBooks Self-Employed. This can be done directly through your account by clicking the Gear icon in the top right corner, then select Feedback.
The following article provides additional info about tagging transactions in QuickBooks Online.
Please don't hesitate to reach back out in the comments below if you have any other questions or concerns. I'll be here to help in any way that I can.