How does a security group sid differ from a per service Sid? I am learning SQL and trying to understand the concept of a SID.

At the link below it states:


  Microsoft SQL Server uses a security group to set resource access
  control lists (ACLs) instead of using the service account directly.
  Therefore, changing the service account can be done without having to
  repeat the resource ACL process. The security group can be a local
  security group, a domain security group, or a service security
  identifier (SID)


and then goes on to say


  For stand-alone instances of SQL Server on Windows Vista and on
  Windows Server 2008 operating systems, service SIDs are added to the
  service group, and the service SID for SQL Server Engine and SQL
  Server Agent is added as a login to the sysadmin server role.


It uses service group and security group interchangeably. Are they the same? ...and linked to this, if a service SID can be added to a security group (which can also be a SID), these 2 SIDs are different right?

I think so, but struggling with the Microsoft documentation.

thanks.

https://support.microsoft.com/en-us/kb/2620201
A SID is a mechanism that assigns privileges to the service itself, rather than to the account under which the service runs.

For Clustered installations:

It enables you to use a domain account with minimal privileges on the box, which also improves your security because people end up knowing the domain service
account credentials. 

For non-clustered installations:

Per-Service SID, it allows you to use Network Service as the service account, improving security by
getting automatic password management, but without the traditional downside of having Network Service accumulate excessive privileges from multiple services.

Refer to : 


SQL &amp; SIDs why we need it and what the hell is it?
SIDs and IDs