Service accounts are used for managing databases. A service account is a user on a SiriDB server, which should not be confused with a database user. A service account has no access to a SiriDB database. The reason for separating these accounts is that a database can extend over multiple servers, while a service account only gives access to a single server.
You can manage the service account(s) and database(s) via the SiriDB Admin tool or using the HTTP API.
Every SiriDB server has already one default service account installed: sa user with password siri.