Assumed Pensionable Pay (APP) is a figure used to ensure your pension is not affected if your pay is reduced when you are absent. For example, for a period of sickness or relevant child related leave. It is your average pensionable pay, calculated using the 3 months or 12 weeks pensionable pay, before the date you received reduced pay.
If you retire on ill health, APP is used to work out any enhancement. If you die in service, the lump sum death grant is based on APP.