The label of an elliptic curve over a number field $K$ has three components, denoting the conductor, the isogeny class and the isomorphism class:
• The isogeny class label normally consists of one or more letters a-z or A-Z; the ordering of the isogeny classes is based on lexicographical ordering of the Dirichlet coefficients of the L-series. For this to be well-defined, a standard ordering is used for the primes of the base field. In the case of elliptic curves $E$ defined over an imaginary quadratic field $K$, the isogeny class label has a prefix "CM" for curves with Complex Multiplication by an order in $K$ itself.
Together these give a label of the form $N.a1$ where $N$ is the conductor label, $a$ the class and $1$ the curve number. Omitting the third component gives an isogeny class label, of the form $N.a$.