The Cartesian product of $A$ with itself $n$ times is normally denoted using superscript notation $A^n$, and this what ISO 31-11 defined as standard. However, ISO 31-11 has been superseded by ISO 80000-2, which specifies the notation as using a subscript: $A_n$, in clause 2.5.7: “$A × A × \ldots × A$ is denoted by $A_n$, where $n$ is the number of factors in the product.”
This looks rather odd and easily confuses with other uses for subscripting. Can this be a typo? I am specifically reading SFS-ISO 80000-2, which contains the subscript notation both in the English text and in the Finnish text. But I have not seen such usage in real life. And BS ISO 80000-2 appears to use a superscript here. What about other national implementations of ISO 80000-2?