Add new fields and methods in standard table in D3FO

Dear All,

I am going to add new fields and methods in Standard Table "SalesTable".

As i know if we wants to work on Standard Objects , we need to use always "Extension". 

In Data Model , we have Table Extensions node and i saw SalesTable.Extension is already there.

I saw in Table "SalesTable" we have an option create extension :

And in Table extensions > SalesTable.Extension are also there :

Kindly elaborate me the best approach to add fields and methods in Standard Table.

Please give me more shed on this.

Thanks!