I don't know about the OS app but in the Legend on paper maps it's at the bottom of the roads, with the explanation 'Other road, drive or track'.
It's easily missed because the illustration in question shows road, track and then back to road, all in one.
As far as I'm concerned if a track doesn't have a Private, Keep Out or No Access sign at the start of it, then I'll walk on it, unless it becomes apparent it's leading to a property or something similar.
My attitude is much the same as Fernman's. Often the Origin point and Destination of a right of way and a track are the same yet the route they take is different. On Moorland this often to do with the availability of earth moving equipment. The old way of finding the best way up a hill was to find a female donkey, or was it a mule or an ass, in heat and take it to the top of a hill, then stake out the trail that a male of the species took to get too the top. The ex-WD bulldozers after after WW2 did away with the need to follow a canny animal.
There are many other reasons why people walked across Moorland, not enough thought is put into how the occupiers of countryside create new tracks and how the could serve the dual purpose of agriculture, other leisure pursuits and walking. The occupiers have had 20 years, if not more in being brainwashed, that de facto use will result in the creation of a RoW.
If more willingness was allowed to merge uses and alter the temerity, as expressed by the OP Toxibunny perhaps we might see an access network that could grow with the times.