Also "There are five distinct inherent differences between BT and CAT tapered tools.
The first difference is that the BT shank holder calls for a ground ID in the retention knob cavity. This accepts a retention knob with a ground pilot underneath the shoulder of the knob. This is important because it keeps the pulling force of the spindle on the centerline of the toolholder to minimize any error in the fit between the toolholder taper and the machine spindle. Also, a retention knob with a ground pilot will repeat to the same position when you remove it and then replace it again. A CAT shank retention knob will not. This is a benefit in maintaining balance in high-speed applications.
The second difference is that the BT shank holder is manufactured for symmetry. Both drive keys have the same width and depth. A CAT shank toolholder has drive keys that vary in width and depth, and, therefore, a correction mark must be made in the smaller keys to compensate for the difference. This makes the BT shank toolholder easier to balance.
The third difference is that the CAT shank toolholder is governed by the ANSI standard, which calls for a relief cut under the flange of the toolholder of 5/8" width. This is a requirement so that there is no interference with carousel-type tool changers. This specification unfortunately weakens the design of the CAT shank. Without the relief cut, in the case of BT, the toolholder can be shortened and a thicker diameter can be maintained under the flange for increased rigidity.
The fourth difference is that all of the dimensions on the BT shank toolholder are in metric. This includes the threads in the retention knob cavity, and the keys.
The fifth and final difference is the slot in the flange of the BT shank toolholder is off-set, plus the flange is thicker. On the CAT shank toolholder the slot in the flange is centered."
The preceeding quote was taken from: