A principal forebearance is where they take part of the loan balance and make it sort of like a balloon at the end of the loan, the forebearance would have to be paid off if you sell or refi the property in a lump sum.
this is from the making home affordable:
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Step 6: If the Front-End DTI Target has not been reached, forbear principal. If there is a principal forbearance amount, a balloon payment of that forbearance amount is due on the maturity date, upon sale of the property, or upon payoff of the interest bearing balance
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