The SOAP API is continuously undergoing updates and improvements, and is available for use in both a Stable version and a Development version.
The Stable version of the SOAP API is fully functional and is not undergoing changes. Its functionality will be maintained in future versions.
Each Development release cycle is 6 months, during which time the Development release is available for use but is undergoing changes and may not stable. You can use the Development version in the same manner as the Stable version, except that the wsdl is different and you must modify your application as needed to accommodate our changes to the API. After the 6-month cycle, the SOAP API becomes the new Stable version, and the next version is created as the new Development version.
Currently, SOAP API v1 is the Stable version. The SOAP API v2 will soon be available to use as the Development version.