Server-to-server applications are the type of applications that work unintended and do not need any interaction with the user. More information about Server-to-server and how to get S2S token you can find here.
Server-to-server integration gives you the option to create Reports (Access to historical information, read-only API key).
In case you use Coinly, CoinTracker, or any other service which can get the data from STEX, they may ask you for a read-only token for access to your reports or your trade history.
To do so, you need is to open the profile settings, then choose the API settings in the list (or open this link):
Сhoose Server-To-Server integrations (1) in the sub-menu, then click 'New S2S Token' (2).
Give a name for a readable token (1) and put in the checkmark next to 'Reports (Access to historical information, read-only API key)' (2).
NOTE: checkmark only 'Reports (Access to historical information, read-only API key)'.
PLEASE be careful with adding any other ticks to this form! Giving other permissions except read-only may lead to money loss, think about the S2S token like about the keys to your house, and do not provide too much access if not needed!