Within the Project Item maintenance screen (Menu path Projects >> Project Items), individual Project Items can have a "locked locally" attribute set. Setting the "Locked Locally" attribute of a Project Item to 'Checked' basically locks the properties of the Project Item so that they CAN NOT be modified locally at the project level when the Project Item is assigned to an individual project. This functionality allows for an organization to define their Project Items with a great deal of accuracy and care, and then enforce the standard definition wherever the Project Item is assigned.
An example: An organization can set up a non billable item such as 'Administration' in the Project Item Master area for use by employees performing non billable tasks. In this case, this items' billing status is not expected to change or vary from project to project. By selecting the "Locked Locally" setting, users cannot change the billing status of this project item when posting time to it.
Note: Communicating with users which Project Items should be used for which type of posting can be done through 'Project Messages' if desired.