Evaluator is an option for Cell which gives the name of the kernel to use to evaluate the contents of a cell.
The default setting is typically Evaluator->"Local".
Evaluator is more often set at a global level or at the level of whole notebooks than at the level of individual cells.
See The Mathematica Book: Section 2.10.2 and Section 2.10.9.
See also: InitializationCell.