The API and attendant WSDL are sometimes assigned a new version number on release. The versions are indicated by a number that is part of the WSDL file name; for example, the name of the WSDL file for version 3.0 is zuora.3.0.wsdl.
Each version might have one or more releases associated with it; each release will introduce bug fixes and enhancements, and sometimes new features, such as new objects or calls or new fields, and therefore new capabilities, for existing objects. However, the version number won't change for those releases; it only changes when a new version is released.
Each WSDL version is backwards compatible with previous versions. You can continue using an older version of the WSDL with your code, or use a newer version without changing your code. However, if you wish to take advantage of new features, you will need to start using the new WSDL and you will need to update your code to reflect the changes.
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