Kerf is the cutting width of the tool used to make your parts. Features smaller than this size may be lost. Also, inside corners have to be able to accommodate the width of the nozzle.
Design your part at exactly the size you want it, and we will do our best to make it that size. There is no need for you to compensate for the cutting path.
Please also read The FAQ on part quality.
We typically cut parts with an abrasive waterjet with a nozzle width of 0.040 inches.
The kerf from the laser beam cut is approximately 0.01 inches.