‘ionumbers’ stands for ‘input/output numbers’.

The package restyles numbers in maths mode. If a number in the input file is written, e.g., as $3,231.44$ as commonly used in English texts, the package is able to restyle it to be output as $3\,231{,}44$ as commonly used in German texts (and vice versa). This may be useful, for example, if you have a large table and want to include it in texts with different output conventions without the need to change the table.

The package can also automatically group digits left of the decimal separator (thousands) and right of the decimal separator (thousandths) in triplets without the need of specifing commas (English) or points (German) as separators. E.g., the input $1234.567890$ can be output as $1\,234.\,567\,890$.

Finally, an e starts the exponent of the number. For example, $21e6$ may be output as $26\times10\,^{6}$.