It's probably a silly question, but I've seen some miniatures listed as HVT in some stores, but I can't find any reference to that term (HVT) in the rules, would someone be so kind as to explain what it is?
An HVT is basically a secondary objective marker. Its a civilian/diplomat/whatever that is interacted with certain missions and classified cards.
So, the tournament rules are in a separate document from the main rules - they set out rules for specific missions. Most people play the missions in non-tournament games as well (because it's boring if the only goal is to kill each other). The mission rules are updated once a year, and are on the company website and called something like "ITS Season 13" (ITS = Infinity Tournament System). You can also install a fan-made Android/IPhone app called Comlog which has all the missions. Most of the missions include an HVT, which is a model each player deploys as a target for some minor objectives. There are a few missions where the HVT is a more important target. So you're not finding it in the base rules because it only exists in the mission rules.
As @QueensGambit and @Urist have stated, a HVT is a civilian/mostly non combatant target with particular uses in the game depending on mission rules. It stands for High Value Target. Many of these HVT minis get proxied as units from armies as some of them are very nice and look great as part of a force. Hope that helps!
Yes it helps, I saw the ITS document, but when I saw "tournaments" I told myself that for now the tournaments are for later, so I didn't pay attention to it. I'll download it and take a look at it. Thank you all three for the replies.