You can check the prices of a season ticket using our Season Ticket Calculator.
Where you see a ticket which mentions the start or end point as 'London Terminals' this means that the season ticket covers you from the first main train station in London that is on the route you are travelling. If you wish to travel any further you will need to add in a London Underground supplement (shown as London Zones 1-6).
For example, searching for Braintree to London Euston, there will be two prices for season ticket: one which will only include the journey from Braintree to London Liverpool Street (shows as Braintree to London Terminals) as this is the first London station on the route from Braintree and another which includes an Underground supplement to get from London Liverpool Street to London Euston (shows as Braintree to London Zones 1-6).
The difference between the two allows unlimited travel within the London zones 1-6, which covers from London Liverpool Street station to London Euston station.