The four tracks that I have raced have had the 4/5 field together, the 3 seperate, and a 1/2 field.
these are Piccolo, Blaine, Kenosha, and Northbrook.
For the most part, in my experience, the Cat. 3 field is the largest. This is where more experienced riders land after upgrading out of 4/5. This field is quite competitive because some riders want to upgrade to Cat. 2. It is fast and furious...some would say 2 Fast and 2 Furious(I love vin diesel!).
An Omnium is the overall point standings for a night.
For instance, all fields at blaine have three races per night. In each individual race the winner receives 5pts, second 3pts, third 2pts, and fourth 1pt. At the end of the night you total up the points given for all three races to get your overall winner. So within the omnium the max points is 15.
Then you take the final OMNIUM Tally and dole out points that go towards your upgrade...
Therefore, you need to get 20 points to upgrade to cat3 at Blaine. So you need to race for a minimum of 4 weeks and win the omnium in order to have 20points to upgrade to cat3.
Now from cat3 to cat2 is a little more difficult. You need to get 25points AND there has to be more than 20 racers in the field to obtain upgrade points. So that is where the difficulty lies.
Another point to be made is that all upgrade points don't carry over to the next season. It used to be that upgrade points carried over in full till the middle of July, or you would lose them. Now the only half of your upgrade points carry over for the entire next season. So I had 16 upgrade points in 2005. I now only have 8 points to upgrade to cat2. Argh.
This method of upgrading to the best of my knowledge is what is used by American Track Racing Association and USA Cycling. But, like mentioned before some tracks do it differently.