Correct. You won't drop any traffic, but whatever doesn't match in any class does fall under the class-default. Your current config guarantees 100Mb of bandwidth to vlan 543, but if vlan 12 were to come in it doesn't guarantee anything for 12 and will just match on the class-default.
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Yes, any unmatched traffic goes to the class-default class. This class is always present, even when not explicitly defined.