When AF-S is selected for viewfinder photography, this option controls whether photographs can be taken only when the camera is in focus (focus priority) or whenever the shutter-release button is pressed (release priority).

Regardless of the option selected, if the in-focus indicator (I) is displayed when AF-S is selected for autofocus mode, focus will lock while the shutter-release button is pressed halfway. Focus lock continues until the shutter is released.