I beleive there was a Standard Bearer article on this a while back. So I'll try and paraphrase it for you:
Back in the day the games designers could make whatever units they wanted and shove them in to the codexes as and when they wanted to. The people who made the models couldn't keep up with the massive amount of things waiting to be built, and people began to get annoyed with it.
Then one day the guy in charge of
GW decided enough was enough and that units could only go into codexes if it had a model already made for it, or if it would get one at the release of the Codex. This worked out pretty well for the model designers as they could finally keep up with things.
However it also meant new units came about less often than the gamers would have liked, as it felt like only pre-existing units ever got new models or entries in the Codexes. As a result the restrictions were lifted somewhat, but under the idea that although new units would be put into a codex withour new models-the capacity for said units to get models would happen. This is seen through the initial first wave of models when the codex is released and then a subsequent release at some point in the future. case in point the Daemons Codex being release over a year ago, and now having the second wave of models being brought out to suplement the range.
In a Tournament I beleive as long as you have converted a unit to be what it is described as in the codex you should be ok. I.e. converting lots of Canis Wolfborn models into Thunderwolf riders should be allowed
Hope that helps, or at least makes some sense