Some verbs are used to talk about things that happen over time.
You might live or work someplace for years. You sleep for hours at a time.
Some verbs happen once and they are done. You open the door, you run out the door -- you do it, and it's over. You generally don't say "I have been opening the door for 0.5 seconds" the way you would say "I have been working here for three years."
Installing relay agents happens once and it's done.
I have installed the relay agents. This is the active form.
The relay agents have been installed. This is the passive form.
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