A truss system is a triangle made up of multiple smaller triangles.
They come in many shapes and sizes. They are made in a factory and delivered to the home site where they are set every 2’ on the walls top plate. They span from one bearing wall to the other which makes all walls in between nonbearing. Theres your roof in a nutshell.
Many Arizonian transplants come from parts of the country which have attics with the capability for storage. However here in AZ most truss systems are not designed for storage. The engineer who designs the truss takes into account three factors, one being the span between bearing walls, two being ” live load,” things like wind, and snow. We don’t have to be concerned with that factor like most other places in the country but the third factor is something called dead load. Dead load is the physical weight of the wood that comprises the truss, the drywall (ceiling) attached to the joist part of the truss are just a couple of examples. Unless the engineer designed the truss system to carry any extra weight like boxes of books, extra lumber left over from a project or any of the things you store in your attic you are changing the structural design and integrity of your roof. If there is no flooring or flooring to the HVAC system only, there is a good chance that your truss system was not designed to store items in your attic. If you decide to ignore this advice at the very least place items above where there is a wall below it. Keep in mind, the previous statement may be smart, but is still not very good advice.