“LinearDirection” option is not documented in the online help for UnrollSrfUV. https://docs.mcneel.com/rhino/6/help/en-us/index.htm#commands/unrollsrf.htm?Highlight=unrollsrfuv
What is the option? I assume it is the direction of the surfaces ruling lines, and “Natural” is automatic selection similar to the same as how the command worked before the option was added.
Version 6 SR20