The fourth argument is, again, item type. Again, I'm wondering what types are understood by the software. Does WEAPON work? CRAFT? What about, I dunno, BRACELET?
Item token, right. Same for reactions: 1st goes item token, 2nd is item subtype, 3rd is matgloss, 4th is matgloss subtype. Items are, obviously, made from material.
Here are item tokens:
http://df.magmawiki.com/index.php/Item_token (very incomplete) and
http://df.magmawiki.com/index.php/40d:Item_tokens (more or less complete for previous versions) for item tokens, and material tokens you'll be able to find on the wiki too =)
I get it up through here. [REACTION_CLASS:FLUX] just means that this boulder has to be a flux stone. But it's got me wondering, what other reaction classes are there? I know there's gypsum. What else? Can I make my own? If so, where do they go?
See e.g. inorganic_stone_mineral.txt , reaction class for matgloss is set there, for example. You can make your own reaction classes, but only for materials, not for items. E.g. stone, metal, or tissues like plant material or skin can have reaction class, but item types/subtypes - sword, toy, door - can not.
Almost everything else you've figured out is exactly true =) Excellent job!
What are the valid entries for the skill argument?
Shaostoul's guide has a list
in this post.
Can I add skills?
No =(