When you tag an Expense as Personal, you'll be able to select a Personal Expense category. There are many default categories to choose from, but you can also create your own custom ones.
To create custom personal expense categories on the mobile app:
Go to the Expenses tab
Tap the gear icon
Tap "Personal Categories"
Tap the + sign in the top right corner
To create custom personal expense categories on the web app:
Click the gear icon in the top right corner
Select "Personal Categories"
Select "+Add button" in top right corner