Hello Krishnnambal,

Has the template explains an External Web Portal is designed for a better User experience being able to adapt to the most commonly used browsers and screen sizes. 

If you look at the CSS of a Reactive web app and an External web portal you can see the ladder has an extra "layer" that provides that adaption .

Hope it helps!

Paulo Rosário

Is an extra CSS layer the only difference between Reactive and External Web Portal?

I've tried to find some more background information to help me to identify if there are  advantages to using External Web Portal.

Documentation has close to no information on when to use this feature..