This mod provide utilities for setting up Unity blend shapes for a character skin and provides APIs for setting the blend shapes values. Mostly used for NSFW purposes (but I won't stop you from using it for SFW mods.)
If you are a (skin) mod developer, you can check out the guide to using this here: https://github.com/maiesen/BodyBlend/wiki/BodyBlend-v0.3-Implementation-Guide