RAM Mounts make over 5,200 different product lines, based on a ball and socket system that has 5 different ball sizes:
"A" sized ball - 0.56" (14.22mm)
"B" sized ball - 1" (25.4mm)
"C" sized ball - 1.5" (38.1mm)
"D" sized ball - 2.25" (57.15mm)
"E" sized ball - 3.38" (85.85mm)
Generally speaking, the RAM Mounts part number will have the ball size noted in it (eg. RAM-B-201U), except in the case of "C" sized balls, just to make things a little confusing! The "C" sized ball is where they started, so part numbers do not include the letter (eg. RAM-201U).
The choice of ball size will depend on the conditions you are going to be in, but also the weight of the unit you are mounting. RAM Mounts recommend the following capacities for each ball size:
"B" sized 1" ball - up to 2.25kg (approx 5lbs)
"C" sized 1.5" ball - up to 4.5kg (approx 10lbs)
"D" sized 2.25" ball - up to 6.75kg (approx 15lbs)
"E" sized 3.38" ball - up to 9kg weight (approx 20lbs)
For most applications you would tend to use a "B" sized ball for GPS, phone, camera, and light tablet applications, or a "C" sized ball for heavier tablets, lighter marine applications, or in rough conditions.