Fiber assignment describes the process of (optimally) allocating available optical fibers to a list of targets selected for observation. The allocation must make allowances for the different observing requirements for each target (needed S/N, number of epochs, cadence, maximum sky background, number of calibrators needed).
While the problem is conceptually similar, many of the details and logistics differ depending on when fiber assignment is done for plates or the Focal Plane System. Refer to those pages for details on how fiber assignment was implementated.