Good luck on the joys of FEI names! But short version:
If it says “C(insert a letter)I” that means it is an international event governed by the FEI, like CAI CDI, CCI, CSI and so on (the A is the driving designation, presumably from some French word that means combined driving, D is dressage, CI is eventing and S is show jumping). You will see some variation of the CxI in all FEI sports. If it has an O at the end it’s an official FEI, like CSIO, meaning championship or other very high level affair.
U25 means a class for drivers under 25 years
Stars are simply the designation of the level:
1* = lowest level international event, equivalent to US intermediate, anything below that is not governed by the FEI (to be utterly confusing, there can be higher level international classes). 1* is just a CT, does only dressage and cones, but you have to do two 1* before you can enter a 2*
2* is effectively “advanced” and is the same as the 3 star when it comes to marathon and cones, although I suppose the course designer could make the course different, but he isn’t required to and all the speed/width and gate rules are the same. The only difference is the dressage test. For singles that mostly means no collected canter work in the 2* . Drivers must complete three 2* before they are eligible to enter a 3*
3* is the highest level dressage test (of you see a single horse canter in, not stop and transition to a collected trot at X you are watching the 3* test.
National classes (in the US, but the prize list looked like CAN follows the format) are as follows:
Training - only trotting allowed on marathon, gates A-D
Preliminary - gates A-D
Intermediate - gates A - E
Advanced - broken into novice and regular advanced (national class may or may not be offered at a CDI, but they use the 2 and 3 star dressage tests) - gates A-F, and
both 2 and 3* do gates A - F
For the lower levels you will often see them go through F (or E for trn/prl) on the way to some other gate because those gates don’t exist for them. But for gates that apply to your division, you must pass sequentially through them, red on right. If you go through C on the way to A, too bad so sad, that’s the Big E for you (eliminated). If you go through a gate backwards and correct it by going through it correctly you get a bunch of penalty points along with all the time penalties you accrue, of you don’t correct it, the Big E for you! Also once you have gone through a gate correctly it is considered dead and the driver may go through it in any direction without penalty (and we do that often).
Enjoy! I have a few friends up there competing and I’m super jealous!