All of those are misleading because the FS segment override isn't specific to an operand. It applies to the whole instruction, which commonly has one place (a memory reference) for the override to take effect. You can have more than one override, but only the last one remains active. Normally you can have an override even if it isn't used. There are a few instructions with more than one memory access; the override only applies to one access and you don't get to choose which one.