In Susanna Epp's book Discrete Mathematics with applications I came across phrases such as "... is an identity for +" or "...is an identity for *", for example
There is only one element of $B$ that is an identity for +
The identity law for + is $a+0=a$, that makes sense. But "$0$ is an identity for +" sounds meaningless to me. What does it mean in the context of these phrases /quote?