This doesn't add anything directly - instead this adds 3 base packages that mods can use to transfer items and fluids between blocks.
Quick overview (for developers):
Attributes: These are available on blocks that implement "IAttributeBlock"
Item and fluid management are both based around 5 similar attribute classes: ItemFixedInvView, ItemFixedInv, ItemInvStats, ItemInsertable, and ItemExtractable.
More details can be found on the wiki. For any other details (or help) using this please either open an issue on github, or ask in the lba channel in the cottonmc discord server.