Depends of which currency is in the nominator of the NER.
E.g. if you are calculating the real exchange rate between the US and the Eurozone
$$
\text{RER} = 0.88 \frac{\text{EUR}}{\text{USD}} \frac{p_{\text{US}}}{p_{\text{EZ}}}.
$$
Since $p_{\text{US}}$ is measured in USD, and $p_{\text{EZ}}$ is measured in EUR, this is the only way RER will make sense. If you calculated
$$
0.88 \frac{\text{EUR}}{\text{USD}} \frac{p_{\text{EZ}}}{p_{\text{US}}}
$$
you would get something that is measured in EUR$^2/$USD$^2$, which would be very strange indeed.