How to automatically build freight unit?

Hi guys,

I'm learning SAP TM, playing around feight unit.

But I'm not clear about the whole picture here.

In fwo type customizing, I can check the "Automatic Freight Unit Building"  flag and assign a default FUBR.

But how the system is going to automatically build the freight unit of a FWO?

And in FUBR creation, I'll specify some strategies and quantity stuff. There's not a field where I can specify which FU type to use.

So how is FWO, FUBR and FU type linked together?

I tried to create a fwo, but there's no fu built. what's the preconditions for the system to build FU automatically?