Consider a house with a gable roof that looks like this:
/\ /--\ / \ / \ / \ ======== | | | | | | +--------+
(the  is a load bearing beam coming out of your screen)
Roof rafters need ties at the bottom to form a truss. In many cases, ceiling joists act as the rafter tie. For long spans, the ceiling joists might overlap on a wall, and will be nailed together, still forming a truss.
But if the ceiling joists are hung from a beam using joist hangers, is that still considered a truss? Or do rafter ties needs to be nailed in above the ceiling?