What is the meaning of blue field in top right hand corner when changing data screen?

It is a tip that shows you what a long press of the top right and button will do on that page. In this case a long press will put you page edit mode. A short press will give you access to the context based menus.

Thanks, aweatherall, for your kind explanations.

I find this feature totally useless and irritating. Why not display a clean screen after switching and, upon pushing the button, just open a submenu if there is one.

The button has two functions, a short press will access the menu, the long press is really a short cut to a specific function to save you going through the menu.

Whether the tip needs to be there is another question.

Tapping and pressing on those icons does absolutely nothing for mine, and why are they different depending on the data page?

The icons are there to show what the long press of the top right hand button does. They do not react to screen touching. They are there as an aid to understand what short cut is programmed into that long button press. What is behind the shortcut depends on the page you are on. Hence the different icons.

For example on most data screens it takes you into edit mode, on the map screen into zoom mode on the elevation page it toggles the X-axis between a narrow view and the whole activity or course.

This likely has greater value for 540 users as they need to do everything via buttons. The 840 user can use the buttons or touch screen to access functionality.