Process Start White List defines on who is permitted to start the process. By default, everyone can start the process. When configured, only those found in the mapping will be allowed to start the process.
Figure 1: Map Participants to Users - White List
It has the same mapping options found in conventional participant mapping therefore there's a wide range of options available.
You may write your own beanshell script or extend using custom participant mapping plugin as well.
Figure 2: Map Participants to Users mapping options