It can store up to 2 billion UU-Matter, as it has the storage capabilities of a Quantum Chest, which it will automatically draw from to craft UU-Matter recipes.
UU-Matter can be inserted from the top or bottom of the device and crafted items can be extracted from the sides. Each time that the UUM-Assembler crafts something, 512 EU is used per UU-Matter used in crafting. Its max EU/t intake is 128 EU.
Neither of these crafting recipes appear to work in FTB Ultimate pack