If a transition in the PDA has a label such as $"$a$"$ or $"\backslash$Delta $",$ what does that mean?

Group of answer choices

It means what character the machine needs to have in order to make a movement.

It means what the machine is writing to the output.

It means what character the machine needs to store before moving to another state.

It means what the machine will act if any of the label character is seen.