Bolt Circle or PDC

General
"0 starts at"
This allows you to set the orientation according to your drawing, design or plan specifications.
 
"coordinates"
This allows you to set the results to match your X - Y coordinate system of your machinery or digital readout.
 
"Incrument"
You would normally set this to
clockwise however if you have to drill from the reverse side then anti clockwise gives you the mirror, this becomes important when the first hole is not at 0 degrees or when you are drilling less holes than are in the PDC.
 
"X and Y origin"
Normally the X and Y origin (datum, zero point ore reference point) is at the centre of the PDC. When the origin is offset from the circle centre then you can give the offset. The offset can be negative or positive. If left blank then 0 is assumed.
 
Normal PDC
The holes are evenly angularly spaced based on 360 / number of holes.
 
"Angle of first hole"
Normally the first hole is drilled at 0 degrees, where you need to first hole to be at some other angle then you can specify that angle, this may be positive or negative. If left blank then 0 is assumed.
 
Arc PDC
The holes are angulary evenly spaced within the arc specified by Start angle and End Angle. The angular spacing is based on arc / number of holes.