Using enterprise fields, organizations can customize project management based on their own structure and processes. All users in the organization have access to a standard set of enterprise fields, allowing operations to be done across entire sets of projects.

Enterprise fields (including their definitions and lookup tables) are stored in the enterprise global template. Values assigned to a task, resource, or the project in an enterprise field, however, are not stored in the enterprise global template. To maintain security and consistency, users must have permission to check out the enterprise global template to change enterprise fields.

How Enterprise Custom Field is used in Web TimeSheet?
Custom Field in Microsoft Project are used for mapping when Project information is transferred from Web TimeSheet to Microsoft Project, it's same as User Defined Field in Web TimeSheet. For example: To map Task Code in Microsoft Project with Web TimeSheet it should be Enterprise Custom Field (Task Type).

How to create Custom Field in Microsoft Project Server?
The Custom Field page in Microsoft Project Server lets you specify the options for a custom field. Use the following procedure to create a new enterprise custom field or edit an existing one.

To create or edit Enterprise Custom Fields:
On the PWA Server Settings page, click Enterprise Custom fields and Lookup Tables.
In the Enterprise Custom Fields section, click the field that you want to edit, or click New Field to create a new field.
Fill out the Custom Field page by specifying the custom field options that you want to use. See the descriptions for each field in the following sections.
Click Save.

Note:
Use the Entity and Type areas to specify whether you want a Project, Resource, or Task custom field, and what data type the field should be.