'Tis a mystery.

A4, etc. gets me too. Never have I seen that term here. The definition I found on Google says it is 216x297mm or 8 1/2 "x 11" (not 8.25"x11.67"- what is that)?
Does A4 differ between Australia and the U. S. and Canada? Going back to Google, I find it listed under European office supplies. Certainly one can't find 8.25"x11.67" paper in the normal stationery or computer supply store.

When I buy paper for my computer printer, I find it is always 8 1/2"x11" = 216x279mm. Or I can buy 'legal size' which is 8 1/2" x 14"

I normally use 20 lb. paper, which the package says is equal to 75g/square meter (I'll take their word for it).

I use 96 (TAPPI) brilliance normally- which in fine print the package says is 108 (ISO) in European measure. Hmmm, ISO- that appears on film packages I buy (400 ISO = 27 oh, my!! Now I am lost).