The ability to tell monzo to have all payments outside of a bill pot take from a specific pot if not enough in main account or take from a specific pot first then if more is needed take from main balance.
Scenario #1
Goto shop and pay by card you have £5 in your main balance but £100 in a reserve pot the payment is for £25 take the 10 from the main balance then the rest from the reserve pot (or the extra is transferred to main balance as part of the transaction similar to bill pots)
Scenario #2
Set reserve pot as default money location or default location for specific transactions (Sainsbury’s for example) so that you goto shop and pay by card it comes out of the reserve pot by default and anything over the amount of that pot is then taken out of main balance or transferred to the pot on the fly from main balance to cover the extra similar to how bill pots extract to main balance as part of transaction
Scenario #3
You make a payment that you know is going to take a while so you put that money into the reserve pot transfer eventually comes through but you don’t have enough in main balance, the extra required is taken from the reserve pot to cover the transaction