Shell describes a stage of completion in which a building or room is not fully finished. The included structure, services and finishes depend on the agreement. The word therefore cannot replace a specific schedule of included and excluded work.
How this term relates to your project
A shell extension may appeal when you intend to arrange some work separately. Check exactly where responsibility changes hands: are services prepared, are substrates ready for later trades, and will temporary facilities still be needed? A boundary described only as basic construction may mean different things to different parties. List the condition in which each element will be handed over and the work that follows.
Discuss who measures for subsequent packages, when those contractors receive access and how completed work will be protected. Interfaces between packages influence the programme as much as the size of each package. Do not compare a shell quote directly with a finished proposal until the omitted work and coordination responsibilities have been identified.