The portion of the potentiometer that touches the inside surface of a panel or other mounting surface. The mounting surface is used as a reference point for measuring shaft & bushing lengths. Shaft and bushing lengths are measured From the Mounting Surface; which is abbreviated as FMS. 

The eccentricity of the shaft with respect to the rotational axis of the shaft, measured at a specified distance from the end of the shaft. The body of the potentiometer is held fixed and the shaft is rotated with a specified load applied radially to the shaft. The eccentricity is expressed in inches, TIR.

The perpendicularity of the mounting surface with respect to the rotational axis of the shaft, measured on the mounting surface at a specified distance from the outside edge of the mounting surface. The shaft is held fixed and the body of the potentiometer is rotated with specified loads applied radially and axially to the body of the pot. The Lateral Runout is expressed in inches.

The total radial excursion of the shaft, measured at a specified distance from the front surface of the unit. A specified radial load is applied alternately in opposite directions at a specified point. Shaft Radial Play is expressed in inches.

The length of the shaft as measured from the mounting surface (FMS) not the end of the bushing.

Dimension "B" represents the correct reference point for shaft length measurements

The length of the bushing as measured from the mounting surface (FMS) 

Dimension "A" represents the correct reference point for bushing length measurements.

An internal or external method of protecting area behind the panel mounting surface. 

A panel seal is typically an O-ring that fits between the bushing mounting surface and the inside surface of the mounting panel.  This type of design usually incorporate circular ridge in the base of the bushing to accommodate the O-ring and to prevent potential deformation of the O-ring while tightening the mounting nut. 

There are also composite washers (Stat-O-Seals)  which consist of a nitrile gasket surrounded by an aluminum (or other metal) washer.  The metal washer prevents potential deformation of the nitrile component while tightening the mounting nut.

The level of protection can be expressed as an International Protection Rating, IP rating (see the Environmental section below), which has specific metrics defining the degree of protection against external environmental conditions.

Internal shaft seals typically incorporates an O-ring which fits into a properly sized groove on the shaft.  The contact between the internal O-ring and the inside surface of the bushing provide a predetermined level of protection from liquids (typically water) or vapor.  Unless specifically designated, an internal shaft seal will not provide protection against liquid or vapor pressure but instead is designed to minimize the migration of the liquid or vapor into the internal working elements of the potentiometer. 

Internal shaft seals DO NOT provide protection from liquid or vapor entering through the panel mounting or the body of the potentiometer.  A separate panel seal is required to prevent liquid or vapor from penetrating through the panel mounting surface.

The level of protection can be expressed as an International Protection Rating, IP rating (see the Environmental section below), which has specific metrics defining the degree of protection against external environmental conditions.

External shaft seals utilize a hollow silicone rubber gasket material that is formed around a threaded nut.  The external seal is placed over the shaft and threaded onto the bushing thereby forming a seal that prevents liquid or vapor from entering the internal working elements of the potentiometer. External shaft seals can provide protection from liquid or vapor entering through the panel mounting, but NOT the body of the potentiometer.

External shaft seals MAY NOT provide protection from liquid or vapor entering through the panel mounting or the body of the potentiometer.  A seperate panel seal is required to prevent liquid or vapor from penettrating through the panel mounting surface.

The level of protection can be expressed as an International Protection Rating, IP rating (see the Environmental section below), which has specific metrics defining the degree of protection against external environmental conditions.

The length of the bushing as measured from the mounting surface (FMS).

The total axial excursion of the shaft, measured at the end of the shaft with a specified axial load supplied alternately in opposite directions. Shaft End Play is expressed in inches.

The maximum moment in the clockwise and counterclockwise directions required to initiate shaft rotation.

The maximum moment in the clockwise and counterclockwise directions required to sustain uniform shaft rotation at a specified speed.

The mass moment of inertia of the rotating elements of the potentiometer about their rotational axis.

A term used to describe a components ability to withstand a liquid solvent washing process that removes flux residue present as a result of wave or other types of soldering.  The board washing process typically involves a brief submission of a printed circuit board after its components have been soldered, into a washing solvent.  Since the submersion into the solvent is only a few inches deep, there is no substantial pressure involved.  This is an important consideration as unless otherwise specified,  the term "board washable" does not imply submersibility into a liquid to any specific depth.  Board-washable and Water-Proof are not equivalent terms.