You can't take a single sat on chain either. Any UTXO amount below the dust limit is essentially unspendable due to the fees required to spend it. Millisats exist in a similar way that sats exist. As an accounting unit. Not necessarily a discrete and fully transferable unit.
I guess, from both your answers, it is useful to divide on lightning network if required but not outside it
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Sats are the only units in the mainnet. Bitcoin itself as a unit doesn't exist. Every transaction is in sats.
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Yes, but millisats are what makes a single sat spendable
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