[over.best.ics]/10:

If several different sequences of conversions exist that each convert the argument to the parameter type, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated the ambiguous conversion sequence.

It might be helpful to clarify the meaning of "several different sequences of conversions". E.g. int -> int is not an ambiguous conversion sequence even though int -> float -> int and int -> long -> int are both "sequences of conversions" in plain-English sense.