Saddle length is the overall distance from tip to tail of the saddle. This measurement should be taken when the saddle is flat or horizontal.
Saddle tip to middle is the horizontal distance from the tip of the saddle to the lowest point on the saddle. This isn't necessarily the true midpoint along the length of the saddle. It is meant to be approximately the point at which the rider is seated.
Saddle tip to clamp is the horizontal distance from the tip of the saddle to the point at which it is clamped to the seatpost. As the saddle is slid fore and aft along the rails, this distance will change.
Saddle thickness, shown below, is not an industry standard measurement but it is used in BikeCAD to define the distance from the lowest point on the top surface of the saddle to the midpoint of the rails of the saddle.