There are several aspects of the API that may change over time.

Version numbers are used to document various kinds of compatibility between different versions.

Major and Minor API Product Version

The product version number for the API governs the compatibility on the source code level, that is, the include files.

Math API has a product version number, which consists of three components: major version number, minor version number, and version qualifier.

Detailed description:

Major Version Number
Changes in the major version number indicate a disruptive change without guaranteeing compatibility with prior versions. Details are documented in the Release Notes.
See MI_MATH_VERSION_MAJOR in Versioning of the Math API.
Minor Version Number
Changes in the minor version number, where the major version number remains unchanged, indicate an upwards compatible change. Old sources compile without changes.
See MI_MATH_VERSION_MINOR in Versioning of the Math API.

Version Qualifier
A string that indicates special versions, such as alpha and beta releases. Final end-customer releases have an empty version qualifier. The compatibility guarantees described here are only given for end-customer releases and do not hold for new features introduced in alpha or beta releases.
See MI_MATH_VERSION_QUALIFIER in Versioning of the Math API.