When setting up your Salesforce Marketing Cloud Account under Account > Integrations > 3rd Party Integrations, you will see something like this:
In short, we need the correct WSDL to properly communicate with your Salesforce Marketing Cloud account. In the WSDL dropdown, you can choose from Default (S1), S4, S6, S7, or Custom.
To determine which WSDL you should use, log into your Salesforce Marketing Cloud account, and look at the URL of your page. You should see something like this:
In this example, the "s7" in the URL indicates that I should use the S7 WSDL from the dropdown. If you see something other than S1, S4, S6, or S7 in your URL, you'll need to use the Custom setting. You can get started with Marketing Cloud Help Documentation, or contact Salesforce Marketing Cloud Support for additional assistance with a custom WSDL.