Hi! There are two types of UCS in Inventor. One is geometry associative (pick three vertices). The other one is purely driven by parameters. Either type is available in Part environment. But, in Assembly environment only parameter-driven UCS is available. So, you simply need to input the X, Y, and Z offset values to place the UCS in an assembly.
Thanks!
Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer