This is what Wikipedia says about Azteca registration. No arab blood is present, but I think Criollos can have a fair bit of Arab blood at times.
Strict breeding codes ensure the success of the breed. The crossbreeding has been scientifically researched to combine only the best of each breed. The horses are inspected twice in their lives: at seven months to obtain a birth certificate, and again at three years of age, to get their registration certificate as breeding stock. The registry attaches the letters A, B, C, D, E, and F to their horses as identification as to their proportions of each breed. The letters are only used for breeding purposes to help the breeders choose horses to produce offspring that are no more than 3/4 Andalusian or Quarter Horse. The "A" level is the most desirable.
Azteca A Registration:
- Azteca A crossed with another Azteca A
- Azteca C crossed with an Andalusian (foal with 5/8 Spanish, 3/8 Quarter Horse)
- Quarter Horse and Azteca C cross (foal 3/8 Spanish, 1/2 Quarter Horse, 1/8 Criollo)
- Quarter Horse and Azteca B cross (foal 3/8 Spanish, 5/8 Quarter Horse)
- Azteca B and Azteca F cross (foal 5/8 Spanish, 1/8 Quarter Horse, 1/4 Criollo)
- Azteca C and Azteca F cross (foal 3/8 Spanish, 3/8 Quarter Horse, 1/4 Criollo)
- Azteca D and Azteca F cross (foal 1/2 Spanish, 1/4 Quarter Horse, 1/4 Criollo)
- Azteca B and Azteca C cross (foal 1/2 Spanish, 1/2 Quarter Horse)
- Azteca B and Azteca D cross (foal 5/8 Spanish, 3/8 Quarter Horse)
- Azteca C and Azteca D cross (foal 3/8 Spanish, 5/8 Quarter Horse)
- Azteca C and Azteca E cross (foal 5/8 Spanish, 3/8 Quarter Horse)
- Azteca D and Azteca E cross (foal 1/2 Spanish, 3/8 Quarter Horse, 1/8 Criollo)
Azteca B Registration:
- Andalusian and Azteca D cross (foal 3/4 Andalusian, 1/4 Quarter Horse)
Azteca C Registration:
- Quarter Horse and Azteca D cross (foal 1/4 Andalusian, 3/4 Quarter Horse)
Azteca D Registration:
- Andalusian stallion and Quarter Horse mare cross (foal 1/2 Andalusian, 1/2 Quarter Horse)
Azteca E Registration:
- Andalusian stallion and Azteca F mare cross (foal 3/4 Spanish, 1/4 Criollo)
Azteca F Registration:
- Andalusian and Criollo cross (foal 1/4 Spanish, 1/4 Criollo)