Alistair Wright

Hello, After a spell living and working overseas I recently applied to pay a shortfall of NICS. I received back the calculations and over different tax years there was a mixture of Class 2 and 3 contributions within HRMC's calculations I could pay. There are instructions within the letter (and website) stating a bank transfer should be made to pay any shortfalls. My question is, if I only wished to pay the Class 2 contribution amounts/years how does HMRC know to allocate my payment to this class and not to make a partial allocation of my payment to the more expensive Class 3 contributions I also have shortfalls with? Thank you