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Great thanks, that's perfect. From what I've read in other posts this seems to have been an issue in the past, but now appears to be solved. So yeah, some clarity for new users who don't have any of this history/context would be great.

Correct - when we first launched PlayFab, all the accounts were in one "bucket". We moved to the Publisher ID model soon after, as it was clear it was needed.

Oh i didnt know that, so for example if Player A creates new PlayFabAccount with a deviceID in title T1 and then downloads T2 from the same publisher, and then login with the same device ID, he gets the same playfabID ?

Does he even share the same PlayerData/Friends etc. ? or does he only share the PublisherData ? 

Are the usage limits per Title or per publisher then ? So for example can a player which plays 3 games from the same publisher have 1000 items in each game or 1000 combined ?

Well first, please note that the limit for items a single player can have in inventory, for our free service is 256 items, not 1000. The limits are designed to be per live title, so a user playing three of your games, all in the same Studio (with the same Publisher ID) could have 256 items in each inventory, in each game. And again, it's worth noting that if you need more than the stated limits for the free service, let us know the specifics at devrel@playfab.com, and we'll work with you on a contract with custom costs for the totals you need.

To answer your other question, User Title Data (plus User Read Only Title Data and User Internal Title Data) is stored per title, while the Publisher equivalents are stored at the Publisher level.

and what about "if Player A creates new PlayFabAccount with a deviceID in title T1 and then downloads T2 from the same publisher, and then login with the same device ID, he gets the same playfabID ?"

How does Playfab knows its the same player ? so when does playfab gives the same playfabID in 2 different titles for the same publisher ? 

There are two key things to consider:

1. Are the titles in the same Publisher ID? Your Publisher ID shows up on the Settings tab for your titles. By default, if you create titles in the same Studio, they have the same Publisher ID. User accounts are created within a Publisher ID's ownership - not a title's.

2. How are you signing the user in? If you're using a new Device ID (they player changed devices) or a sign-in method that the user has never used before (if the first title didn't have Facebook login, but the second title did, for instance), then those are separate accounts. Once a user is signed into an account, you can use the Link... API calls to attach many other authentication mechanisms to it. Any and all authentication mechanisms linked to a user account will cause the user to be signed into that account when signing into any title which has the same Publisher ID.