Specifies which account the report server NT service should execute under. When omitted or when the value is empty string, ...

Specifies which account the report server NT service should execute under.  When omitted or when the value is empty string, the default built-in account for the current operating system.
The username part of RSSVCACCOUNT is a maximum of 20 characters long and
The domain part of RSSVCACCOUNT is a maximum of 254 characters long.