This question is more of a high level. An external system is trying to connect with my Salesforce app. The external system- ABC, is using the partner wsdl to update the record on Salesforce. ABC has the id of the record and field api name that needs to be updated. I see an operation named Update in partner WSDL and assume that ABC can call this operation to update the record.
How would ABC know to update a particular field of the record using the update operation? If I have added new field to Salesforce should I provide the partnerWSDL again to the end system? I think No.