One form of mental accounting is where you give yourself separate budgets for things like food, entertainment, gas, etc. It’s suboptimal because these separate budgets make you less flexible in your consumption plans. For example in a month where there are many attractive entertainment offerings, you are unable to reallocate spending away from other goods in favor of entertainment.
But it could be understood as a second-best solution when you have memory limitations. Suppose that when you decide how much to spend on groceries, you often forget or even fail to think of how much you have been spending on gas this month. If so, then its not really possible to be as flexible as you would be in the first-best because there’s no way to reduce your grocery expenditures in tandem with the increased spending on gas.
That means that you should not increase your spending on gas. In other words you should stick to a fixed gas budget.
Now memory is associative, i.e. current experiences stimulate memories of related experiences. This can give some structure to the theory. It makes sense to have a budget for entertainment overall rather than separate budgets for movies and concerts because when you are thinking of one you are likely to recall your spending on the other. So the boundaries of budget categories should be determined by an optimal grouping of expenditures based on how closely associated they are in memory.
(Discussion with Asher Wolinsky and Simone Galperti)