The envelope method is one of the oldest budgeting systems. The original idea: when you got paid, you split the cash into physical envelopes —“food”, “fun”, “transport”— and only spent from each envelope what was inside.
In its digital version, each envelope is a category with a monthly limit. When it empties, you stop that category until next month. A finance app with category budgets implements the same idea without handling cash.
It works well for people who lose track when paying by card and is useful to keep specific variable expenses in check: eating out, subscriptions, leisure.