Complex sentence: Both purebreds and mixed breeds are allowed to compete, but must be at least 15 months old and have a "permanent card", or "P-card", which gives the "lowest allowable Regular height" a dog may jump at.
Split into simpler sentences: Both purebreds and mixed breeds are allowed to compete, but must be at least 15 months old to compete. A dog's "permanent card", or "P-card", signifies the "lowest allowable Regular height" a dog may jump at.