Isn’t that the million dollar question! Which races you choose to enter is one of the major choices you will make as a stable owner. Generally speaking, your goal (assuming your goal is maximum performance and not say, limit testing) is to enter your horse into races that are MOST suited to their attributes and preferences.

We consider Gaia Mode for the Golden Gate Allowance.
An example case
We see here that the Golden Gate Allowance is an 8 furlong, right-turning turf race. The weather prediction is a 50% chance of Cloudy weather, meaning that there is a 1/2 chance that at the time of the race the Condition of the track will be “Soft”. Gaia Mode is a horse which is fully suited to the race by Preference (provided the weather ends up being Cloudy), preferring soft, right-turning turf tracks. An 8 furlong race is a middle distance race, meaning her Speed and Stamina will be important primary stats.
In short, Gaia Mode is a horse which is fairly well suited to the Golden Gate Allowance – the only major thing left to contend with is the level of competition she will end up facing when the horn sounds.