When running the Get-MailboxStatistics cmdlet, two of the attributes returned are AssociatedItemCount and ItemCount. But what exactly are the differences between the two?!
As one would expect, ItemCount is the total number of e-mails, contacts, etc. items available and visible to the end-user on the mailbox. Basically, everything the users sees.
On the other hand, AssociatedItemCount are special messages in the Associated Table of a folder. They are most often rules and other special messages like the MRM FAI message. Basically, things that are “associated” with a folder, usually other file-like objects not visible to users such as IPM.Configuration.RssRule, IPM.Configuration.AutoComplete, Outlook Rules Organizer or IPM.Microsoft.MigrationStatus.
These are not visible to the user, only through MFCMapi for example (right-click a folder and select Open Associated Contents table).
Folder Associated Information (FAI) is a collection of message objects that are stored in a folder object and are typically hidden from view by e-mail applications. An FAI Message object is used to store a variety of settings and auxiliary data, including forms, views, calendar options, favorites and category lists.