Hi Qtron,
Don't know, guess depends on the diameter, pitch, material etc. I see that most people are using 1000+ PPR so that may say something.
This "External, Internal, Manual" switch sets how auto feed works for threading and cutting when you have motor on the cross slide screw.
Like when you want to cut thread in fully automatic mode (with motors on 2 axes) you choose whether you want to cut internal thread or external thread (it defines which direction cross slide is moved on each pass). Same for multi-pass machining in automatic mode. So if you don't have motor on the cross slide - this switch will not be of any help. At least that's how I understand it.
Post #3 here
https://www.chipmaker.ru/topic/11808...omment-1930024 has detailed description of how modes work and refers to this sub-modes as "Ext", "Int", and "Man".
Again, I think it shall be "Manual" unless you have second motor on the cross slide.