It is essentially a future perfect form because of 'until'. Whenever we use a point in time as a reference - when, as soon as, after, until, etc. - we use present perfect or present simple for the future, as the other part of the sentence tells us it is future. I will call you when I arrive. I will call you when I've arrived. I will wait until you finish it. I will wait until you've finished it. With these time references, they can only refer to future or past, and never the present. So we use past for the past and present for the future. The other clause tells us the time. After all suitable candidates were interviewed, we made a decision. After all suitable candidates had been interviewed, we made a decision. After all suitable candidates are interviewed, we'll make a decision. After all suitable candidates have been interviewed, we'll make a decision. So we use past/present perfect/simple, and the difference in meaning between perfect and simple is very similar to how it is in Russian. I will call you when I arrive. - Soon after I arrive - probably to tell you that I've arrived. I will call you after I've arrived. - Some time after I arrive - just for a chat or something.

The model to remember is "Until something happens". Notice that present tense ("happens") is used. Your example is the same. You cannot use the past tense ("until the candidates were interviewed"). You must use a present tense: "the candidates have been interviewed". That tense is called "present perfect". It is a present tense, not past, a fact that causes much confusion. That is why it is called PRESENT perfect. The use of the past participle "interviewed" might confuse you, but it functions more like an adjective than a verb. It is not part of the verb. If you replace "interviewed" with another adjective, you will see the point better: "until all suitable candidates have been happy".

The decision will be made as soon as all the candidates have been interviewed, so it is directly relevant to the outcome of what has just taken place. The past tense would be used if you were reporting on the event which had already occurred : ‘The decision was made when all candidates had been interviewed’, and the second clause is in the past perfect as the event [candidates were interviewed] precedes the decision about employment.

Here we need to use the present perfect because it hasn't yet happened (it's something that will happen in the future) "until candidates were interviewed..."? would need to be followed by "no decision WAS made about any future appointment." I hope this helps. Let me know if you have any questions

To put it simply it's a perfect tense form sentence, the use if indicates until something happens, no decisions will be made, other examples can be until I have finished my assignment, I won't go home. all Candidates haven't been interviewed yet, so until they are no decision is made. we can also use future perfect here, a decision will have been made after all candidates are interviewed.