What do the 'Grades' mean for pistons?

I was looking for the correct part number for a piston for a 4.2 V8 Supercharged engine in an L322 and there are three on offer... Grade 1, Grade 2, Grade 3. What does this mean? Is it another way of expressing whether it is oversized?

ive not worked on an l322 but on all other engines bores and pistons are made to a nominal size but in reality size varies between blocks and pistons by a thou or so ,so blocks and pistons are size graded to give best possible fit , if you look on a rover v8 block you can see size(grade) letter stamped on the outer jacket of cylinder,for repair of used engines pistons werent supplied graded even though piston size still varied as your expected to rebore cylinder to your new piston size