It's the list that tissues appear in the workshop after butchery. (I think I left out eyes and liver).
Anyway, the order matters, because stacks of meat are broken (just like bones) and reactions don't use them correctly, unlike stacks of other things. Instead, if you bring a stack of meat of 500 to a workshop and have a reaction (like [GRIND_MEAT]) with a reagent amount of, say, 5, it will use 5 of the meat and the other 495 will be destroyed. Unlike in reactions with non-bady-part stacks, where 495 would (correctly) be preserved OR the reaction would scale by multiples.
To get around this, I made many reactions in different denominations, so like "grind 1 meat" "grind 5 meat" "grind 10 meat" "grind 20 meat" "grind 100 meat" etc. and they auto-activate in descending order, so it will choose the largest available option, and thus only waste small %ages of meat, despite stacks being broken.
However, since organs are also grindable (ba dum tish!), what happens is that if you have, say, 20 meat only 1 pancreas left, it will perform the "grind 10 meat" reaction and grab the pancreas first, since it's first in the list, then the 20 meat stack, and make 10 meat total, wasting the other 11 meat.
But if meat were listed first, then it would grind 20 meat, use all the available meat stack, then grind 1 meat, and use the pancreas, leading to less waste.
In short, I want the largest stacks first (meat, then the biggest typical organs, all the way down to eyes last) so that mathematically, you tend to get minimal waste.