Caitiff generation is determined just like any other clan. For each point you invest in the Generation background, you lower your generation, which raises your max bloodpool by 1.
The difference between 13th and 10th Gen is really minimal if you ask me. However, at 9th Generation you can begin to spend 2 blood per round. If you have disciplines that require blood expenditures, this is rather nice.
However, going down to 9th generation requires 4 dots in said background.