question

Ok, so i just checked out that if you did a login with LoginWithCustomID and after that make a call with LoginWithFacebook i get a new Facebook User. And vice versa. this is perfect and even simpler.

But then the question is which is the main account.

In my case i make a game which is playable on iOS android and Facebook. So i do LoginWithFacebook and once i got this i link all deviceIDs to this account?

But still what if people want to sign out of Facebook and play in guest mode? Then i need a second type of login, right?

To be clear, the recommendation is to only have one account. Sign the player in the first time using something like Device ID (if possible), so that it's zero friction (or as close as possible), and then add other authentication mechanisms via the Link... API calls. That way, you can sign into the account using any of those methods, allowing players to sign into their account event if they change devices.

So, to use your example, when the player starts your game, you would check to see if he already has an account by using LoginWithIOSDeviceID, with CreateAccount set to false. If there's no account, ask the player if they want to sign into an account or just start playing. If they want to sign in, offer all the options you have. If they choose to start playing, use LoginWithIOSDeviceID, with CreateAccount set to true to set up the account. Later, incentivize the player to add another login mechanism (free in game VC, etc.), so that you have other ways to sign them in.

We also have a blog post on best practices for login, here: https://playfab.com/blog/first-impressions-count-best-practices-friction-free-player-authentication/.