>When EXEC executes a SQL Agent job with a syntax like this :
>
>

>EXEC [Something\Something].[msdb].[dbo].sp_start_job @job_name=N'Something'
>

>
>What would represent the first two place holders? Is it server name and instance?

That would be:

EXEC [ServerName\InstanceName].[msdb].[dbo].sp_start_job @job_name=N'Something'

I saw it several times when one server has installed more than 1 SQL Server. IOW, the instance name serves to identify each of them.