This is coming from the perspective of someone who has not yet defeated Calus.
So there's tokens, keys and encounter chest rewards ... now correct me if I'm wrong, it works like this.
1) End encounter chest, chance to reward raid gear, but not guaranteed. If you don't get gear, you have a chance at an underbelly key, and if you don't get a key, you will always get tokens.
2) If you get a key, you have to enter the underbelly and find the apropriate zone, kill the watchers before they alert enemies, and then open the chest in that zone. You have a chance at a piece of gear from that chest, otherwise you get tokens once again.
3) If you don't get gear from either the encounter chest, or the underbelly chest, then you will have received tokens, but can't use them until you defeat Calus and activate the tower bot to cash them in.
Is that all correct?
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Not only is what you stated correct, but even once you do beat him and unlock the vendor for the tokens - it gives you weapons and armor BELOW your light level just like any other vendor. How's that for raid loot? You can go do public events and get better loot than from the raid.