Any asset code ending with a ".S" represents an On-chain staked balance. For example, XTZ.S represents On-chain staked Tezos while XTZ are non-staked Tezos.
Any asset code ending with a ".M" represents an Off-chain staked balance. For example, XBT.M represents Off-chain staked Bitcoin while XBT are non-staked Bitcoin.
You may notice staked balances when looking over your Ledger history.