Did you look it up in file_changes.txt?
Christ, I've been mucking around with this game for four years, and only today do I learn that it comes with a detailed changelog.
Cheers, Milo!
For the record, file_changes reports:
"improvement products can now target products that are given [PRODUCT_TOKEN:<token>] assuming the target is above the improvement in the raw def"
Also, the scrollmaking and bookbinding reactions both make use of it, as well:
[REACTION:MAKE_SCROLL]
[NAME:make scroll]
[BUILDING:CRAFTSMAN:NONE]
[REAGENT:sheet:10000:SHEET:NONE:NONE:NONE]
[REAGENT:scroll rollers:1:TOOL:ITEM_TOOL_SCROLL_ROLLERS:NONE:NONE]
[PRODUCT:100:1:TOOL:ITEM_TOOL_SCROLL:GET_MATERIAL_FROM_REAGENT:sheet:NONE]
[PRODUCT_TOKEN:scroll]
[IMPROVEMENT:100:scroll:SPECIFIC:ROLLERS:GET_MATERIAL_FROM_REAGENT:scroll rollers:NONE]
[SKILL:BOOKBINDING]
[REACTION:BIND_BOOK]
[NAME:bind book]
[BUILDING:CRAFTSMAN:NONE]
[REAGENT:quire:1:TOOL:ITEM_TOOL_QUIRE:NONE:NONE]
[HAS_WRITING_IMPROVEMENT]
[CAN_USE_ARTIFACT]
[CAN_USE_LOCATION_RESERVED]
[REAGENT:binding:1:TOOL:ITEM_TOOL_BOOK_BINDING:NONE:NONE]
[REAGENT:thread:15000:THREAD:NONE:NONE:NONE]
[NOT_WEB]
[PRODUCT:100:1:BOOK:NONE:GET_MATERIAL_FROM_REAGENT:binding:NONE]
[PRODUCT_TOKEN:book]
[TRANSFER_ARTIFACT_STATUS]
[IMPROVEMENT:100:book:PAGES:GET_MATERIAL_FROM_REAGENT:quire:NONE]
note -- right now, the above will auto-pull any writing from the quire and place it in the pages improvement
[SKILL:BOOKBINDING]
So it looks like you use PRODUCT_TOKEN to assign a reagent name to a product, in order to improve it later on in the same rxn?
That does beg the question of why it's included on all the adventurer crafting stuff, but knowing Toady, they could just be a placeholder for things to come.
Would make some new rxns and test it out, buuut... it's half-past midnight after the end of a long work week, and there are half-a-dozen people getting all social-like and/or high directly under my room, so just can't be arsed right now. Sorry!
In the meantime, it looks as though omitting it won't break anything. Good enough!
EDIT: Lies! Could be arsed after all.
And yeah, that's exactly what it's used for - improving a product during the same reaction in which it gets made. No more and no less.
It's not accepted as a product token in other contexts, unfortunately, which means no crazy fractal reactions.