Whenever I am drawing hatching I get a question about automatically creating a centre line. I always say yes to it, but wonder what is it all about?
You are probably correct to say 'yes' to automatically create the centre line. I usually do so.
The normal way of drawing hatching, whether triangular or area, is to: (a) draw one side and press return/enter to finish that side, (b) then draw along the 2nd side, again pressing return/enter to finish, (c) then say 'Yes' to the question about automatically creating the centre line and the item is drawn/finished.
All the hatching created in KeyLINES, KeySIGNALS and AutoSTRIPE are done so with 3 base lines; side 1, side 2 and the imaginary centre. The imaginary centre is required to measure the 45 degrees for the diagonals. With most hatching and especially triangular it is not exactly 45 degrees at the sides but somewhere in the middle. The imaginary centre line that KeyLINES, KeySIGNALS and AutoSTRIPE uses, is where it calculates the exact 45 degrees and that is why it is necessary.
If you want to have this line 'off centre' then it is possible to say 'No' to the question and then draw along the this line where you want it positioned. This may be more common with chevron style hatching. However I have to say I usually just say 'Yes' even if I want it off centre because it is quicker, and then I will simply adjust the centre line using the grips immediately after it has been drawn.