The Buildpack API specifies the interface between a lifecycle program and one or more buildpacks.
A buildpack only ever implements one Buildpack API version at a time.
The implemented Buildpack API version can be found in the buildpack.toml file in the buildpack’s root directory,
or in a label on a buildpack package.
A lifecycle may (and usually does) support more than one Buildpack API version at a time.
The supported Buildpack API version(s) can be found in the lifecycle.toml file in a lifecycle tarball,
or in a label on the lifecycle image.
A lifecycle “supports” a buildpack if they both declare support for the same Buildpack API version in format: <major>.<minor>.
Two buildpacks of different Buildpack API versions can participate in the same build, provided they are both supported by the lifecycle.
You can read the complete Buildpack API specification on Github.