On the Mission Planner - Configuration - ArduCopter screen, you can set ch6 and ch7 using the drop down selections.
Ch7 can be set to RTL.
I don't know what "lock" is?
Thank you Arashi :)
I find to put ch7 in RTL.
I think that locked is "Stabilize" (that's my Y6 to Stabilize automatically).
Sorry for my English, as I am French, I use sometimes google translation.