The label of a genus 2 curve over $\Q$ has the form $N$.$a$.$D$.$n$, where

- $N$ is the conductor,
- $a$ is a string that distinguishes this isogeny class among those with the same conductor),
- $D$ is the absolute discriminant, and
- $n$ is a number that distinguishes the curve among other curves with the same absolute discriminant in the same isogeny class.

In particular, $N.a$ is the label of the isogeny class of the Jacobian of the curve.