So it's basically a XML file that defines sticker packs and stickers as png file...

dudus on Sept 2, 2021 | parent | context | favorite | on: Show HN: I made an app to draw stickers for iMessa...

So it's basically a XML file that defines sticker packs and stickers as png files. No notes on size constraints, image formats, installation, distribution, sharing across apps. Seems like an early stage standard.