To provide the best assistance, I'd like to gain some additional insight into your question. Where exactly are you trying to categorize your mortgages? Once I know this information I can provide you with the accurate steps to categorize it. If you're trying to setup categories in your Chart of Accounts, I've included the steps to do so below. Check it out:
Go to Gear, then Chart of Accounts.
Hit the New button to create a new account.
In the Account Type drop-down, pick your account type. Do the same for your detail type.
Name the account, then add a description.
If this is going to be a sub-account of another category, tick the Is sub-account of box and then select the parent account from the drop-down.
When your finished filling out the form, click Save and Close.