Hi nl322,
Being able to categorize your transactions is an important part of managing your books. Sub-categories could certainly be a useful tool, but is not currently an available feature in QuickBooks Self-Employed. Feel free to leave feedback for our product development team in the app by going to the Gear icon and scrolling to Help. When the chat opens up, type feedback and what you'd like to share and it'll be passed to the team.
Reach out again if you need anything else. :)
