How to Handle Unpaid Interest

When there's an underpayment or $0 payment recorded on a loan in Bryt, the Principal dues get left as part of the balance (there is no Outstanding Principal category). 

As for Interest, you will have the option to set it as outstanding interest or set the due amount to the principal balance. 

To do this, you'll want to either underpay the interest portion or record a $0 payment: 

Then you'll want to click on 'Enter Manually' on the following screen: 

By default, it gets added to outstanding interest as you'll note above. However, by clicking Enter Manually, you'll be prompted a drop-down selection for 'Add to Principal Balance' ad shown below. 

Once you click on 'Finish', you'll see the ending balance reflect the addition:

As for unpaid late fees and unpaid lender fees, those would be added as outstanding late fees and outstanding lender fees, you'll see them reflected as balances on the loan's summary page.
Impound is particularly unique because if it's unpaid, it's forgone. You'll need to create a new impound fee or double up on Impound in given pay periods to "Catch Up" on any unpaid Impound dues.