Post jigs are used for location from a bore.
Plate jigs are sometimes made with legs to raise the jig off the table for large work.
Plate jigs mainly consist of a single bush plate with a provision for location and the clamping of workpiece. So the drill bushes guide the drills.
Jig will only hold the workpiece and guide the tool. It will not hold the Tool.
Jigs and fixture increases accuracy. Jigs are holding device and fixture are generally used for bigger work piece. Jigs are preferred over fixture in drilling because they also help in guidance the tool.