>should seed.dbasupport.com and sid should be same .
Do they have to be? No. Should they? That is a matter for personal preference. I prefer they are the same.

>When these names are created .
The SID is created when you create the database. seed.dbasupport.com is just an alias. You create the alias when you are ready to have clients connect to the database.

>when we connect to database we connect it by >userid/password@host string ----------
>the host string refers to seed.dbasupport.com or sid name
sqlplus system/[email protected] (in most cases. You can mess around with your sqlnet.ora file and specify that dbasupport.com is your default domain thereby bypassing the need to put dbasupport.com for every connect.)