Postdoctoral fellow appointments, as referred to in 5.3.3 of MIT Policies and Procedures, are not appropriate costs for organized research activities (research project WBS) of the Institute. The primary purpose of the postdoctoral fellow appointment is the development of the individual and not the advancement of a research project. See Policy reference below.
Non-federal awards in particular could mention the term Fellow in the context of recruiting postdoctoral level staff to work in their research programs. When we do have major agreements where the sponsor is supporting both an organized research program and a fellowship award program (two distinct purposes), RAS creates a research WBS for the organized research program and a fellowship WBS for the fellowship award program.
Please note, however, that where grants are made for the primary purpose of postdoctoral research training (e.g. NIH T32 training grants), consistent with A-21, the grant is established in an organized research account and postdoctoral fellow appointments are allowable charges to the research account.