What credits must be specified to connect SFB and SSRS?

Question

SQL Server

Mikes in the configuration documentation https://docs.microsoft.com/en-us/skypeforbusiness/... has a hint:
On the Specify Credentials page, in the User name box, type the domain name and user name of the account to be used when accessing the Monitoring Reports (for example, litwareinc\kenmyer). If you do not use this format (domain\user name) an error will occur.

The question is, what are these credits? Just a user? To MSSQL? To SSRS Base? To the SSRS service? What's going on?