The Facebook structure can be intimidating when you go from your personal account, to a broader variety of shared Pages/ Ad Account and other objects, so let's try to get a general view of how these things work.
Short description: It's you.
It is usually registered as your name and surname and connected to an email. It can be personal as this one or a business one. You can actually have both a personal account and a business account, and you may also be able to login into them with different emails (but it is still you ;) ).
What kind of information AdEspresso reads from users: AdEspresso use the FB User to present you the list of Pages and Places you are admin or advertiser of. Most important of all, on connecting an user, AdEspresso shows you all the Ad Accounts that are managed by the user and allows you to import them within the platform for your advertising purpose.
We also use it one-time for setting your Locale and matching as much as possible your data format.
Facebook Ad Account
Short description: This is where you put your CC on AdEspresso
These are the accounts you use for doing advertising, you put your business info for the invoice here and select the time-zone and currency you will be using. Your list of advertising accounts is available here for personal user, and here for business accounts. Ad Accounts are necessary and used to create campaigns. Each campaign belongs to a single Ad Account and cannot be moved from that Ad Account.
Ad Accounts have an ID (immutable) and a name (optional, mutable and typically you have a pre-set Ad Account that has the same name as your user).
What kind of information AdEspresso reads from Ad Account: a tons of them. AdEspresso uses the Ad Account info to show the proper currency for your campaigns (campaigns published with that Ad Account), presents you an updated list of your Custom Audiences and lets you create new ones, same goes with the conversion pixels. It is used to update the list of promotable applications and its timezone is used for your campaigns' dayparting. Not to mention, the list of your campaigns.
Custom audiences, saved audiences and tracking Pixels
Custom audiences, saved audiences, and tracking pixels are objects connected to advertising accounts. This means that if you imported 3 different advertising account, custom audiences connected to the first account won't be available if you are using the third account, even if both belongs to you and are both connected to AdEspresso. So, if you aren't finding your Custom audience (or Pixel tracking there), after ensuring the CA exists and is ready to be used, you should ask yourself “Does it belong to the account I'm using right now?”
Here is a simplified diagram about the structure:
Let's try with more practical examples:
Q: Can FB user B promote Page 1?
A: No he cannot, Page 1 is only connected to Fb User A.
Q: Can Fb user B use Custom audiences of Ad Account 001?
A: Yes he can. Custom Audiences are accessible via Ad Account and Fb User B has access to it.
Q: Can Application A be promoted with the Ad Account ID 002?
A: No, it cannot. Applications are connected to the Ad Account and only Ad Account ID 003 can promote Application A.