GET_MATERIAL_FROM_REAGENT has two arguments:
1st) The id of the reagent to get the material from ("log")
2nd) The specific part of the reagent that you are targeting. A bit confusing, but the reason for this is that all of the default reactions (almost all of them) that use this tag, use it for creature body parts, like skin into leather, for instance. Therefore, the "material of this corpse" isn't specific enough to let the game know you're talking about skin in particular. So there's a place for an ID here to specify which tissues. And then that lines up with matching tags in the tissue definitions, to help it target the right part of the corpse. Otherwise it would just always use bones.
In cases where there are no complications like that--such as when the reagent is simply a log--the easiest syntax is just "NONE" which is the wildcard for "any of the materials of that reagent" but there is only one material for a log, so it always chooses that one.
Technically, if you wanted to, you could put in "wood_mat" there and then go add "MATERIAL_REACTION_PRODUCT:wood_mat" to the wood part of every tree definition, and it would also work (maybe??), I think, but that's a lot of unnecessary work versus just using a wildcard variable to refer to the only choice of material.