Similar to part 3, but not as extreme. It is most likely just going out with someone. It doesn't include making out(which is part 2.5) or touching the privates of your other(which is part 2.8).
Anything sexual that isn't sexual intercourse can be defined as part 2.xx. The more sexual it is, the higher the digit.
Brian: "So we totally did part 2.5 last night, it was awesome."
Wes: "Woohoo! We got up to part 2.8, but Meg wanted to stop."
Brian: "We got up to part 3."
Brian and Wes: "Stfu."