Another question: so it means the transaction may be dropped if the fee is too low?
It's possible that most of the nodes drop your transaction, but there is no guarantee that it won't be confirmed.
As long as there is no other confirmed transaction invalidating that transaction, there's a chance that the transaction is confirmed.
If you have broadcasted a transaction and you want to make sure that it won't be confirmed, you must create a new transaction using the same input and invalidate that.
As long as the transaction you broadcasted is valid, it can be confirmed. It's possible that the transaction is confirmed even if you broadcasted it months ago and even if your wallet doesn't show it anymore.