The Projected tolerance zone modifier is generally used for threaded holes and holes that will receive a pin (usually a press fit) and the designer is concerned that the screw or pin projecting from the hole might interfere with the surface of the clearance hole in the mating part. In these cases it is more important to know where the stud or fastener will be rather than where the hole or tapped hole is.
The projected value is usually the maximum thickness of the mating part or the maximum height of the pin or stud.
The specified value for the projected tolerance zone is a minimum. The specified tolerance applies only to the projection of the hole indicated by the projected tolerance.
The symbol is placed in the feature control frame as of the 1994 Standard. Prior to this, the symbol was placed in a separate frame under the feature control frame.
The amount the tolerance is projected is usually written next to the symbol. For clarity the direction and height of the projected tolerance may be indicated with a heavy chain line drawn closely adjacent to an extension of the center line of the hole.