A “portable” mortgage comes with an option that allows the borrower to transfer the mortgage to a new property (typically subject to credit approval and a property appraisal).
The benefit is that the borrower avoids paying a penalty to break the mortgage early. The homeowner also gets to keep his/her present interest rate after moving.
If a borrower moves and buys a more expensive house, many lenders will permit a “port and increase.” In other words, the lender will port the mortgage and increase the amount. The borrower gets the lender’s current rate on the “new money,” and that rate is then typically blended with the rate on the existing mortgage.