For this €* you need the GLOBal pass-in whatever form it comes (NOT the 1 country-which is valid in as the name says). Travels IN GB-to see such famous places as Leeds, Hull, York, or Shakespeare Stratford or the Lakes-its all free (CAN RES if wanted for free at a counter In GB).

Yes, all seats €* MUST be REServed-at considerable extra cost-mostly 30€, and you also have to hurry to do that as its often fully booked (or at least the quota for passholder) days advance-if not weeks in hi-season. A ticket on a bus (coach for brits) may cost less for the usual trips from LON to BRU or PAR.

As most newbees choose FRance-going onward there-most only stick to those superfast TGV for which FRance is so famous (there are ALTernatives enough)-these also all require RES-and they also have quota. Going the other way-BRU-tne on to NederLand or DE=Germany-its only optional, or if done only a very few €

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

It is simply terminology that is confusing you.

Eurail offer 2 versions of the Global Pass - a Flexi pass (X days in Y months) and a continuous pass (any z days in a row). 

Both passes are identical in their operation and included trains but the flexi pass needs careful management to get best value for your travels. The continuous pass (e.g. 15 days) is more expensive but is ideal for somebody following an open itinerary or wanting to simply hop on and hop off throughout their journey.