What it is
E635 is a 50/50 blend of E627 (disodium guanylate) and E631 (disodium 5'-inosinate). The E631 half is most often produced from fish or meat (see the separate E631 entry); the E627 half is usually microbially fermented and vegan.
Why it matters
Used widely in savoury snacks, instant noodles, stock cubes, gravy granules, crisps, and processed soups. Provides the same "umami boost" as E631 alone but at lower cost.
What to do
Because the E631 component is almost always animal-sourced, treat E635 as not vegan unless the manufacturer specifically confirms plant or microbial sourcing for both halves. A small number of vegan brands use fully-fermented E635 and label it as such.