SL is a Linux release put together by Fermilab, CERN, and various other labs and universities around the world. Its primary purpose is to reduce duplicated effort of the labs, and to have a common install base for the various experimenters.
The base SL distribution is basically Enterprise Linux, recompiled from source.
Our main goal for the base distribution is to have everything compatible with Enterprise, with only a few minor additions or changes. Examples of items that were added are Alpine, and OpenAFS.
Our secondary goal is to allow easy customization for a site, without disturbing the Scientific Linux base. The various labs are able to add their own modifications to their own site areas. By the magic of scripts, and the anaconda installer, each site is to be able to create their own distributions with minimal effort. Or, if a user wishes, they can simply install the base SL release.