This may be a dumb question but I'll ask anyway. When you log in on Air Canada's website and go to your profile page, there's a preferences tab that among other things allows you to enter your membership number for additional FF programs you participate in. If I do so, then how does this work? If I enter my UA Mileage Plus info, will miles I fly still be credited to my Aeroplan account by default? Or will flights on AC metal credit to Aeroplan and flights on United credit to Mileage Plus? Or am I going to mess things up altogether by entering this info, and I'm better to leave it blank?

Sorry if it's a dumb question, and/or been asked previously, but I was not able to find an answer via searching old threads.

Thanks