When you create a new transaction and choose to memorize it in personal finance software, all the details associated with that transaction such as the total amount and category are associated with the payee for that transaction. When you enter the payee into the register in your personal finance software later on, all details are automatically entered into the register.
Memorized transactions are particularly helpful for cutting down on data entry in personal finance software when entering recurring transactions for payees you do business with often and weekly, monthly or yearly income and expense entries.

