Accounts

Different accounts are meant to be completely separate.

Each account has its own tables, whose names contain the "accountID".  For example, "BA_3_notes".  The only shared tables are login-related tables.  Everything is in same database.

Accounts are sometimes referred to as "sites" (though they are on the same domain name.)

Users

Each account can support multiple users. 

Each user (in a given account) is assigned consecutive userNumber values, starting from 0.

User number 0 is regarded as the owner of the account (and is automatically an admin, over-riding any setting in the  admin  flag in the database.)  Any other user may be designated as an admin by setting the admin  flag in the database for that user.

Analogy: accounts are like different apartments; users are like housemates sharing the same apartment

Permissions

Each account separately maintains its own permissions.

Within a given account, user permissions are assigned to Categories and to individual Content Items (such as notes or images.)

Permissions are handled by the permissions.php module.