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I have a Many-to-Many editor linked into my Affiliate Company entity (company/org). This M-to-M relationship is so we can class them as multiple types as a multi-selection.  

Now in the UI, this translates to a middle step in authoring for the entity. Here I can select 1-N values from the Collection Picker, which is great for creating the many-2-many links.
 

In this case though the mapping table containing the ID’s from both parents also has it’s own attribute, IsPrimaryType, to tell us what the *main* type for the company is. I can expose the value on the Collection attribute list, but it does not want to let me check the box to set the attribute to True for the one I want…do you know if there is a way to get the interface to allow this to be edited…is it just read only because it requires an override or something else?

You cannot modify the values of any attribute of the mapping entity in the many to many picker.

You will have the configure a separate stepper on the mapping entity itself to change its values.