This option will be unavailable if one of your Activities is Shift Dependent.
Stop the work and hold the Activity in suspension until another Resource can be found (perhaps because one finishes work at another Activity).
As above but don't let any remaining Resources be used on suspended Activities. Suspended Activities should not be considered for restart until the next shift change.
The behavior tab also lets you control what happens when shifts overlap. The default is that Resources that are available in two shifts that overlap are available to the extent of the largest availability of the two shifts. You can change this assumption to summing together the two availabilities.