Rafael Dos Santos,
Since, I work on these parts on a regular basis, I will to explain to you how I look at them.
Stuffing box: Stuffing box is a traditional way to secure packing in place. A packing Gland is used to compress or release packing from stuffing box. The ID of stuffing box contains the rolls of packing while the OD sits against head/cover of pump.
Seal chamber: This is basically a stuffing box modified and changed for mechanical seal upgrade. It can be a completely new design for containing mechanical seal in case or cover of pump.
Seal cavity: Seal cavity is simply the bore where mechanical seal sits. Like you have probably heard about gasket cavity. Gasket cavity is the area where gasket sits. Seal cavity is not a part but a bore on seal chamber where seal sits. Hope this helps!