The symbol for an inverter (logic gate) is usually the one shown below left. But I have sometimes seen the symbol shown below right. (See, for example, the last image in [this answer](https://electronics.stackexchange.com/a/452448/8090).) What does this difference in symbols represent?

[![inverter symbols](https://i.stack.imgur.com/gw9iM.png)](https://i.stack.imgur.com/gw9iM.png)