http://sparrowa7io5pz6ud3ehqzosvepbxbxt2zphmkjsylp2zgxooko23pqd.onion/docs/spending-privately.html
When you create a Bitcoin transaction, you combine one or more UTXOs as inputs, providing the funds you are looking to spend.
When the transaction is broadcast and mined in a block, all of these UTXOs become spent.
In other words, they are not reduced in value, but completely consumed, and can never be spent again.