It's determined by initiative. Unlike in WHFB, in 40K there is no initiative bonus for charging troops (with a few exceptions: Banshees, BA and certain CSM are the ones that come to my mind right now).
This older version of the new assault rules is only slightly different from the final version, most aspects are included. The new "trial" assault rules are official, they are listed as a "recommended optional" rule. Most tourneys use the new rules.
However, you'll notice that independent characters and tyranid monstrous creatures got toned down a lot (they can't target specific models anymore), while normal squad characters benefit of these rules (you can target ICs and TMCs because these count as units on their own, while the squad characters can't be targeted). Crossfire doesn't exist anymore, now there is something similar which is extremely hard to use though. If a unit falls back after close combat and the advancing enemy "catches" them, then it doesn't count as destroyed anymore. Instead, the close combat continues as normal.
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