A more thorough explanation:

PACTUM

Pactum is a Latin term used in Roman and civil law to refer to an agreement or convention that falls short of a contract. It is usually a less formal agreement, also known as a pact, and can take various forms.

• Pactum constitutae pecuniae: This is an agreement for a fixed sum of money. It is an informal agreement to pay an existing debt, either one's own or another's, at a fixed time. The agreement was enforceable by a praetor, and Justinian extended it from money to debts of any kind.
• Pactum de non petendo: This is an agreement not to sue. It is an agreement in which a creditor promises not to enforce the debt.
• Pactum de retrovendendo: This is an agreement concerning the selling back of an object. It gives the seller the right to repurchase the item sold within a certain period and at a fixed price.

The examples illustrate how pactum can take different forms and serve different purposes. Pactum constitutae pecuniae is an agreement to pay a debt, while pactum de non petendo is an agreement not to enforce a debt. Pactum de retrovendendo is an agreement that gives the seller the right to repurchase an item sold. These examples show that pactum is a flexible term that can cover various types of agreements that fall short of a formal contract.