This is an abbreviation corresponding to four numbers which identifies and distinguishes each Work Unit. The first number specifies the Project, which is the protein under study. A single project number is made up of thousands of individual Work Units. The remaining three numbers identify the specific part of the project that has been assigned exclusively to you. Work is rarely duplicated (except when a computer fails to complete an assignment in a timely manner) so every set of PRCG numbers is unique and only assigned to one person.
Technical details of the PRCG numbers and how Work Units relate to each other are covered under the more advanced Simulation FAQ.