Holds folder unique ID.

A Long value.  

object.UID [= value]

The UID Property syntax has these parts:

As you might have read so far - UIDs (unique IDs) are very important for IMAP protocol. Each folder has his own UID which is never changed for his lifetime. If you recreate folder with same name, his UID will not be the same - and clients will see it as completely different folder. Why is this important? Because clients may keep some local (local on their side) information about message flags (seen messages..), and they probably use UID to make difference between folders.

When you create new Folder using Add method, wodImapServer will immediately assign new UID to the folder. If that's really first time this folder is created, just leave this value as is. But - if you add folder intentionally later on, try to set this property as you have it saved from previous sessions - otherwise clients will have problems accessing the folder.

When talking about UIDs - folder UID must be unique among all folders for the same user. Other user may have same UID - because most probably clients will access other users using other client applications.