agile Principle

PBI: Product Backlog Item

IIndependentThe PBI should be self-contained, in a way that there is no inherent dependency on another PBI.
NNegotiablePBIs are not explicit contracts and should leave space for discussion.
VValuableA PBI must deliver value to the stakeholders.
EEstimate-ableYou must always be able to estimate the size of a PBI.
SSmallPBIs should not be so big as to become impossible to plan/task/prioritize within a level of accuracy.
TTestableThe PBI or its related description must provide the necessary information to make test development possible.

Source: INVEST Mnemonic