On Pmt Account

Key On Pmt Account in Contingent Claim refers to an optional text label that may be used to identify the name of the "account" that holds the "amount" produced by the On Pmt Fn function.
The words "account" and "amount" are in double quotes because their meaning deviates from the usual monetary meaning, as explained below.
Note, case and blanks are important, except of blanks to the left and right which are ignored.

In more detail:

Each time during the simulation when the function On Pmt Fn is valuated, the numerical result of that valuation is stored in an account uniquely associated with the function On Pmt Fn.
The label here simply serves to identify that account.

The account represents a monetary account only when the currency in On Pmt Currency is specified.
Otherwise, it represents a register that holds the numerical output of the function On Pmt Fn as soon as that function is valuated during the simulation.