The term porting is simply another way to say transfer. It is widely recognised when discussing Residential Mortgages, however, perhaps not so much when in connection with a Buy to Let Mortgage. In principle, the meaning of the term remains the same regardless of the type of mortgage. What a portable mortgage enables you to do is transfer the terms and conditions of the outstanding loan to another property being purchased should you look to sell the current property it is secured upon.
Do note however, that although a lender will permit the terms to be transferred this will still be subject to you fulfilling their criteria at that time. Should this not be the case, and they are unable to assist, they can still decline your application resulting in you being unable to port the scheme and having an early repayment charge applied when the mortgage is redeemed should this be applicable.
Virtually all Buy to Let Mortgages are portable however, this does mean that some will not be, and although it is not quite so common place, this is a requirement for the borrower when in connection with a Buy to Let Mortgage, do ensure you check this when reading your Buy to Let Mortgage terms and conditions.