The mechanism inside worksharing that splits one central model into logical, independently-editable chunks - shell and core, interiors, site, MEP - so multiple people can work on the same file at once without constantly stepping on each other. Every element belongs to exactly one workset, and each user can choose which worksets are open and visible in their own session, which is also how a large, heavy model stays workable: closing worksets you don't need speeds up your local session considerably.

Worksets are about dividing ownership within one file; if you need to bring in a genuinely separate file, that is what Linked Models are for.