In a Consolidated Schedule, volatile IDs will show repeated numbers representing a task’s ID in a subproject. This creates confusion and it is difficult to search a task by ID. Therefore, when using a Consolidated Schedule (Master Schedule), you need to differentiate one project from another. One way to do this is to use a custom field to identify which task in a subproject you are working with.
A Product or Project Schedule UID (PS-UID) is used to uniquely identify a task by a subsystem or project identifier and a Unique ID. This PS-UID smart-number is a concatenation of, usually, 2 alpha-characters Project or Subsystem and a 5 character numeric field comprised of a tasks UID (task field). A project or subsystem designator can be hard-coded into the PS-UID formula or, if multiple projects or subsystems exist within one project file (stand-alone project file), you can use a custom text field to specify a task’s project or subsystem. In this example, we will use Text29 to define a custom PS-UID. When we’re done, the PS-UID field will look as follows in Fig. 1:
After selecting Formula button in Fig. 2, the Formula window appears, as shown in Fig. 3. Enter the formula as shown in Fig. 3. In this example, “EM” is the 2 character Project Subsystem identifier.
Remember, “EM” can be replaced with another text field if you have multiple subsystems or projects within the same project file. (E.G. Text18, for Subsystem, as shown in Fig. 4)
Note that the UID field is formatted to 5 characters for visual uniformity in field size.
Fig. 5 shows how the PS-UID looks after you select OK and insert Text29 field.
When used in a consolidated file, each task will be represented by its respective subproject file designator as shown in Fig. 6.
 Master Schedule: A Microsoft Project file that consists of multiple embedded Microsoft project files (a.k.a. Master Project and subprojects or Master Shell).
 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.