There are different generations Savannah cats. It is indicated by the letter F and a number. The number tells you how many generations the specific Savannah cat is removed from the Serval, its ancestor.
An F1 cat is a first generation descendant and has a Serval parent. Usually the Serval parent is male, but it can be a female as well. An F2 is a second generation descendant and therefore has a Serval grandparent. An F3 is a third generation Savannah cat and has a Serval great-grandparent, etcetera.
Sometimes the generation number is accompanied by a letter. They have the following meanings:
A: One parent is a Savannah, the other parent is not a Savannah.
(An example is an F1 Savannah cat who has a Savannah mother and a Serval father)
B: Both parents are Savannahs. At least one parent is an A and at least one grandparent is not Savannah. (Example: An F2 kitten: both parents are Savannahs, say an F5 male and an F1 female. The parents of the F1 female are a Serval male and say an F3 Savannah female.)
C: Both parents and all grandparents are Savannas, but at least one great-grandparent is not a Savannah.
SBT: The kitten has at least three generations Savannahs above him and is regarded as a ‘pure’ Savannah.
F1A x F5A = F2B Kittens
F2B x F5B = F3C Kittens
F2A x F5C = F3 B Kittens
F2B x F5 SBT = F4 SBT Kittens
F3C x F5C = F4 SBT Kittens
F4C x F5C = F4 SBT Kittens