API and Product Versioning

The API version does not correspond to the product version. For example, the WHMCS API was version 2 when WHMCS 9.0 released.

Version Independence

If WHMCS updates or changes the WHMCS product to resolve bugs, the product version changes but the API version may remain the same.

If WHMCS alters either the behavior or structure of the request or response, the API version increments to reflect the incompatible changes.

Backwards Compatibility

WHMCS may continue to support preexisting API versions for backwards compatibility. For example, WHMCS would support /api/v2/ and /api/v3/ when incrementing the version, so calls to API v2 continue to function normally.

Update Channel Example

For example, your system might run the Beta update channel release of WHMCS 9.1.0 and WHMCS API v2.

If you upgrade to the Stable update channel of WHMCS 9.1.0 and receive a fix for promotional code calculations, you would now run a different release of WHMCS 9.1.0. However, the API version would remain the same. This is because the change did not alter the behavior or structure of the API's requests or responses.
If WHMCS had added a new feature to allow multiple promotional codes, it would increment both the product version and the API version.