Our favorite guardian trainer, Dimitri, has a couple of tats on his neck, but it's not just because he's a bad boy. Rose describes his marks to us when she first meets him, and she recognizes them immediately: "they looked like two streaks of jagged lightning crossing in an X symbol. One for each Strigoi he'd killed. Above them was a twisting line, sort of like a snake, that marked him as a guardian. The promise mark" (2.6). Immediately, we know Dimitri means business.
So he got one mark for every Strigoi he's killed. It's a sign of honor and a huge accomplishment for someone to have these marks on their neck, because everyone gets what they symbolize right away. It's kinda like someone saying don't mess with me just by the tattoos on their neck. In fact, molnija actually means lightning in Russian, so we're guessing the tattoos have a lot to do with his lightning speed and reflexes in being able to defeat not one but (count 'em) six Strigoi. One thing's for sure: we wouldn't want to start a fight with Dimitri.