Twin thread or Coarse drywall screws? What difference? etc

12mm PB going up on ceiling, what difference does it make wether twin or coarse?

Are twin/coarse intended for different purposes?

I had to shim the underside of joists with 9.5 ply strips, panel pin and pva fixed, so I'll probably use longer than the usual recommended 38mm (not that 38mm screws actually are 38mm). Maybe 45mm?

Thank you
Dain

 

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