Question: As I’ve known, a to-infinitive is used as an adjective referring to something that hasn’t happened yet, but I found a to-infinitive is used to refer to something that has already happened-“the largest bird ever to exist.” Can a to-infinitive refer to something that has already happened?

“Argentavis magnificens was among the largest flying birds ever to exist.”

<https://en.m.wikipedia.org/wiki/Argentavis>

I see no difference between this and the example below.

[“There’s food to be cooked yesterday”](https://ell.stackexchange.com/questions/294033/there-s-food-to-be-cooked-yesterday)

Answer: 
"when told to" indicates a command or instruction. For example, on a cruise ship, one may be instructed to put on a life vest "when told to" by a crew member. A student may be instructed to only open the test booklet "when told to" -- meaning do not open the test booklet until the command is given.

The sentence above treats the pump as a person who can be "told to" -- i.e. given a command. The pump receives some sort of signal that "tells" -- i.e. commands -- the pump to turn on. The PLC monitors the sensor to determine if the pump complied with the command.
"When told to" implies that the PLC has done more than simply supply power to the pump or activate a relay. The word "told" makes me suspect that the pump has some level of smarts (e.g. microcontroller) and that the PLC is communicating to it that it needs to turn on using an instruction set that both the PLC and the pump understand.

Edit: My answer is based on more than what is written or shown in that infographic. I'll update this answer later but, for now, see my comment below.