I created a subform that I would like inserted into a CI whenever a specific category is selected. I created the subform and the insertion marker on the CI form. When I create the rule, I needs to enter in a Source field. There is nothing to select from the Source Field drop down and I can't type anything in there. What am I missing? Where do i create or reference the Source Field?
You can create generic relations only between EntityRef/LookupItem items (like code to code, code to service call, template to form etc).
You cannot create generic relation between static type fields (like numbers or strings or dates). Since Source ID (if it is the default Source ID field) is string, you cannot create a generic relation to that.