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