Occlusion: when an object disappears and might come back
Occlusion happens when an object is partly or fully blocked from view for some stretch of frames, like a pedestrian walking behind a parked truck, or two people crossing paths and briefly overlapping. When the object reappears, the tracker faces a genuinely ambiguous decision: is this the same object that was just hidden, meaning the existing identity should continue, or is it a new object that happens to be entering the scene at a similar spot, meaning it deserves a new identity.
This gets harder the longer the occlusion lasts and the more the scene changes while the object is hidden. A tracker typically has to lean on cues like predicted motion, where the object was heading before it disappeared, and appearance, what it looked like, to make this call, and both cues degrade the longer the gap is.