When we configure a set of email campaigns to go to an audience of only those interested in a procedure, MML will be looking at the Procedure field of the lead to determine which campaign audience to assign.

When you get leads from Contact Us or similar forms on your website, the Procedure field in MML can be mapped by your webmaster to a selection someone makes on that website form.  If there is no such mapping, then any information from the form is captured in the Patient Comments section.  In that scenario, the person would be assigned to the Default (generic version) campaign until/unless someone manually updated the Procedure field.  

This is a partial screenshot of a lead in MML showing the Procedure and Comments fields.

This is an example of a set of procedure-based campaigns, including a Default one.