There are a number of factors which determine what specific races a horse is allowed to enter. The age and sex of the horse are taken into consideration, as well as its racing experience, its previous racing record and its handicap rating. The requirements for entry for Flat races and Jump races is not identical, there are differences - some subtle, some not so subtle - for equine athletes of either persuasion.
A lot of races have strict policies when it comes to the age of the contenders. Two-year-old horses for example contend exclusively with horses of their age group, these races are classified as 'Juvenile Races'. As two is the youngest age at which a horse is permitted to race, the age restriction ensures that the new race horse has competition at an equal experience level. On the other hand many races are restricted to horses over the age of four, excluding the inexperienced colts and fillies and reserving the action for seasoned race horses to compete on even levels.
While most races for mature horses, aged four and older, are open to both sexes; there are some races which are open to males or females only. Especially with the younger horses, this takes care to even the odds, as a colt (a male horse under four) will be superior in strength and speed to a filly (a female horse under four). There are often Filly equivalents to major races, which are often slightly less valuable, as female race horses are rare in comparison to their male counterparts.
Maiden and Novices' races are open only to relatively unexperienced race horses. A maiden race is open only to a horse that has never won a race - again to keep it fair. A novices' race is open to horses which have not yet one a race in the present season, but may have had victories in previous seasons. No horse is allowed to enter a handicap race before it has run at least three other races, the performance in which will determine the horse's future handicap rating.
Once a horse has accumulated enough experience to obtain a handicap rating, it is allowed to enter into handicap races. However, the rating of the horse determines the level of racing in which it may participate. Lower Class races are open to horses with low handicap ratings, the top Classes are open only to those who have proven their talent and received a high handicap rating.
Although the handicap rating determines the extra weight a horse has to carry in a race, it is not actually based solely on the horse's weight but rather the strength of performance. The best Flat racing horses can have a handicap around 130, the champion jumpers have a handicap rating ranking in the 180s. When a horse with a handicap rating of 110 - for example - competes with a horse rated 120, the higher ranking horse is burdened with an additional 10 pounds of weight to give the lesser ranking horse an even fighting chance.