A countersink consists of a conical hole that is coaxial to a cylindrical hole, where the angle of the cone is determined by the fastener to be used. The purpose of a countersink is to allow a fastener, typically a flathead screw, to sit slightly below the surface of the part.
A spotface is a very shallow flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole. Its purpose is to provide a flat mounting surface for mating parts, such as washers or pan head screws.
The counterbore symbol is used on a drawing to indicate that a counterbore hole feature is required. A counterbore is a flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole.
The Tangent Plane Symbol is used in conjunction with GD&T surface controls to indicate that a specific control is applied to a theoretical tangent plane simulated by the high points of irregular tangible surface rather than the actual elements of the surface themselves.
The Independency Symbol is used on drawings to declare that the requirement for perfect form at MMC or LMC is removed and the form tolerance may be larger than the size tolerance. This symbol only exists in the ASME Y14.5 standards, not the ISO GPS standards.
GD&T Rule #1, also known as the Envelope principle, states that the form of a regular feature of size is controlled by its “limits of size." Limits of size, or otherwise known as size tolerances, can be seen in many forms. A few of them are symmetric, unilateral, and bilateral.