Andrea, there was a link to the FAQs in the red box over your post. 

General Help: https://cycle.travel/advice/map  (full menu on left of page)

FAQs https://cycle.travel/advice/map/faq

In relation to the app, Help is available under the 'Settings' menu. 

There is also a long YouTube video that may well be worth the effort of watching

YouTube: https://www.youtube.com/watch?v=MB-Rvt5qBFo&t=21s

You may also find the answer to your question using the 'Search' option on the top right of this screen.

As to your original question, I'm afraid I don't understand.

If I ask CT to direct me from A to B and I cycle to B, then turn around and cycle back to A I cannot expect the app to understand what I am doing. 

I would stop the app at B, then ask it to plot a route back to A. That may be as simple as opening A-B, and in Settings selecting 'Reverse Route'. (That's Android, IOS may be slightly different - you haven't said which version you're using.).

On the website, CT will happily generate a loop route, usually with a different return leg. Once saved, that route will be available in your app.