phpDocumentor uses the rules as layed out in the Semantic Versioning Specification version 2.0.0 [SEMVER].

What is considered the “API”?

The “API” as intended with the Semantic Versioning specification consists of three parts:

# Commands, any command exposed by phpDocumentor is considered to be part of the API. As such commands or their options
may not be removed or changed without increasing the major version number.