Sling Money is not in a position to provide tax advice. You should consult your own tax advisor to understand whether you have to report Sling Money transactions to your local tax authority and whether they may represent gains (or losses) on your taxes.
When you add money to Sling Money, you are swapping your currency for a digital euro (EURC, if you created an account in Europe) or digital dollar (USDP, if made anywhere else). When you remove money from Sling Money, you are swapping it back. In some countries, this may be a reportable virtual currency transaction.