• Independent: The PBI should be self-contained, in a way that there is no inherent dependency on another PBI.
• Negotiable: PBIs are not explicit contracts and should leave space for discussion.
• Valuable: A PBI must deliver value to the stakeholders.
• Estimable: You must always be able to estimate the size of a PBI.
• Small: PBIs should not be so big as to become impossible to plan/task/prioritize within a level of accuracy.
• Testable: The PBI or its related description must provide the necessary information to make test development possible.

Source: INVEST mnemonic