I have a list of data ("Courses") with a flow configured on it that allows users to "request" that course. They cannot create new items in the list or delete items from it, so I created a custom permission level called "Edit only" (which does exactly what I need). So, the users are in the Visitors group of the site, and that group has "Read" and "Edit only" permissions to the Courses list.
I then added that Courses list as a Run-only user for the flow. However, the users were not able to see the flow in order to initiate it. According to https://docs.microsoft.com/en-us/business-applications-release-notes/april18/microsoft-flow/sharepoint-lists-libraries:
...all users that have read/write access to the Document library automatically have permissions to run the flow from the Flow menu in SharePoint.
So, I would suspect that if I just gave the group "Contribute" permission to the list, it would work, but then they'd be able to add items (which I don't want). I guess by "read/write access", they actually mean "Contribute" level permission, and what I've created is "read/edit access"?