Add Enterprise Custom Lookup Tables :-
Consider using Add Enterprise custom lookup tables for any Enterprise Custom Field for which standardization of data is the most important factor. For example, it might not be a good practice to let users enter an arbitrary value in a custom Status field. One project manager might enter Started, and another might enter In-Progress, both indicating that the project has begun and is underway. Without using lookup tables, it is difficult to standardize terminology in your organization.
Here I am describing you step by step how to Add Enterprise Custom Lookup Tables
Step 1: Go to Project Web App home page, click Project Web App Settings.
Step 2: click Enterprise Custom fields and Lookup Tables in the section Enterprise Data.
step 3: click New Lookup Table to create a new lookup table in the section Lookup Tables for Custom Fields.
Step 4: type a name for the lookup table in the Name box for a new table.
Step 5: Fill out the Custom Lookup Table web page and Click Save