Complex sentence: The Ontario Sire Stakes is a category of Canadian Thoroughbred and Standardbred horse race restricted to horses of either sex sired by a stallion standing in the Province of Ontario for his entire season the year of conception and registered with the Canadian Thoroughbred Horse Society for that season.
Split into simpler sentences: The Ontario Sire Stakes is a category of Canadian Thoroughbred and Standardbred horse race. For Thoroughbred horse races, the race is restricted to horses of either sex sired by a stallion standing in the Province of Ontario for his entire season the year of conception and registered with the Canadian Thoroughbred Horse Society for that season.