the mortgages are arranged similarly to the deeds - my own intro to the Ballaugh mortages on CD (only a few are however summarised ) - is:-
Mortgages recorded pre 1723 are numbered in a different sequence - as Old Mortgages, the sequence number does not reflect the date. Like NSS the mortgages for all parishes are numbered in a common sequence for each court up to 1798 - up p to 1756 the mortgages were numbered in sequence after the deeds but after this time were numbered in their own sequence.

Sometimes these mortgages are assigned to others - indicated as assignments.

The usual form is the Manx Mortgage in which the Mortgagee advances a sum of money to the land holder - the Mortgagor - and then takes possession of the land, the benefits earned from working the land provide the interest on the capital loaned out. There are a few mortgages in which the capital is loaned on the security of the land but the Mortgagor retains possession but pays interest (usually at 1s in the £ loaned or 5%) - there was a legal limit to interest rates set under Usury laws. The mortgage was taken out for a limited term though sometimes with an option to continue from year to year with mutual agreement - if at the end of the term the capital could not be repaid the Mortgagee took possession of the land. Often the cover will indicate that a mortgage had been redeemed - indicated as Cancelled

I'm sure that Nigel will have a better explantion - I'm no lawyer. However the mortgage has a redemption value - the sum loaned + any interest - this is the equity mentioned.

In looking up mortgages there are 2 books in museum library (and also on mormon films) arranged like NSS + SSS which cover up to c.1814 - I havn't checked if the system changed then or if this is just the cutoff date for the indices