Chainstay length is one of the key dimensions in bicycle frame design. While it has always been an option to directly input chainstay length, many have chosen to control the rear end by rear wheel gap instead. By setting a rear wheel gap, you can always be sure that no matter how the seat angle, seat tube curvature or bottom bracket drop change, you'll have the required clearance between the rear tire and the back of the seat tube or bottom bracket shell.
In BikeCAD version 17, we now have a third option for controlling the rear end, and that is to specify the distance between the rear axle and the back of the seat tube. Unlike rear wheel gap, this option is only concerned with the top length of tube which is in line with the seatpost, not any curved sections below or the bottom bracket shell, either of which could theoretically be closer to the rear axle than the section at the top.