When referring to money or equivalents, the box can refer to the cash register (AmE) or till (BrE) or possibly wherever money is stored
safe deposit box
It should not be confused with a vault which has a different context.
An equivalent meaning to box is bank which is when a waiter or waitress carries money on their person without going back to a central register
Each wait-staff carries their own bank which is then reconciled at the end of the night.
A technical use of the word box is where one keeps their stock (AmE) or share (BrE) holdings
When you short against the box you use your holdings (as collateral) to sell against it(self)
Also, when security houses used to manually transfer investment certificates, those operations were in an area generically called the box, where security was very high.