What is the difference between a valet key and master key?

I have 5 keys for my 05 Xterra, one is a valet key, and one of the programmed keys does not work. (ie. the car won't turn over when I try this key) The Nissan dealer says I can only have 5 keys total, and they need all the keys in order to program a new key. Is this true, and if so, how does it all work?
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