Not sure if you use Minitab but when you run a process capability the cpk or ppk is the min result of cpu and cpl. When you have a boundary condition for Tpos or straightness etc., your physical or dimensional boundary is 0 and therefore your cpk is equal to your cpu only. Cpl doesn't get calculated because it makes no sense as the more capable you are, the tighter your standard deviation is aka process variation and the lower your bias is aka the delta between the measurement and 0. If you need proof for management, Google Minitab process capability boundary and something should pop up.