It is a process of coding, categorizing and evaluating economic outcomes. For instance, people are known to make notional mental accounts, i.e., accounts for education of children or savings, etc. Even within investment accounts, there is a safe account designed to provide for the “rainy days” and one account that is typically the risk taking account designed to make money grow faster.
However, theoretically, speaking money should be fungible across accounts because money in one account is as good as money in some other account. In practice, however, money is not found to be fungible across accounts. For instance, fall in value of investment from the safe account causes more pain than an equivalent fall in value in the other risk taking accounts.