The Unique ID field contains the number that Microsoft Office Project automatically designates whenever a new task, resource, or assignment is created in the current project. This number indicates the sequence in which the task, resource, or assignment was created, regardless of placement in the schedule.
There are several categories of Unique ID fields.
Data Type Integer (integer field: A type of field whose content is a whole number. Examples include the ID and Unique ID fields.)
Unique ID (task field)
Unique ID (resource field)
Unique ID (assignment field)