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Tribal is a card type (like creature or instant), not a supertype (like legendary). The type carries no rules that affect how it’s cast or how it behaves on the battlefield; it just allows the card to have creature types without being a creature.

If you put Bound in Silence onto the battlefield without casting it, perhaps using the “recruiting” ability of a card like Amrou Scout, it will enter attached to a creature. You choose that creature as you’re putting the Aura onto the battlefield. This doesn’t target that creature, so you could have it enter the battlefield attached to a creature an opponent controls with hexproof, for example. If there’s no creature on the battlefield it can be attached to, it stays in whatever zone it was in.

If Bound in Silence becomes attached to a creature that’s already attacking or blocking, the creature continues to attack or block as normal.