The rate_type property describes the nature of the interest rate being applied in the rate property. This is useful in determining future cash flows
A fixed rate type, as defined in the Bank of England's Mortgage Lending & Administration Report (MLAR) Definitions:
means the rate of interest is fixed for a stated period. It should also include any products with a 'capped rate' (i.e. subject to a guaranteed maximum rate) and any products that are 'collared loans' (i.e. subject to a minimum and a maximum rate). Annual review or stabilised payment loans should be excluded (since the purpose is merely to smooth cash flow on variable rate loans);
A variable rate, as defined in the Bank of England's Mortgage Lending & Administration Report (MLAR) Definitions:
includes all other interest rate bases (i.e. other than those defined above as 'fixed') applying to particular products, including those at, or at a discount or premium to, one of the firm's administered lending rates; those linked to Libor (or other market rate); those linked to an index (e.g. FTSE) etc. However if any such loan products are subject to a 'capped rate', then treat as 'fixed'.
A preferential rate type is often a temporary lower interest rate compared to what the bank would normally charge as an introductory offer or a reward for loyal customers.