Well, I never bothered to try and puzzle it out, 'cos I never used it.
Being one of those old-fashioned pilots who knew roughly where he was and where he was going I would use that old magic mental 'DR' or the database generated map centre-line (dotted white) or approach final track (a beautiful Magenta) if I needed them and worked out my track miles and therefore required/actual energy levels as my vectors changed . A dying skill, I fear, but at least you don't have to agonise over what
all the coloured lines mean, nor push too many buttons when you should be thinking. All the F/Os seemed to be hell bent on extending the c/line but I suspect they didn't really have a clue what it meant. With 2 CDU's and 2 HSI's I just used to let them get on with it. With 1, I would break their arm.
There is actually a neat way to get a continual 'miles to run' on the 737 by putting your 'aiming point' at the top of the legs but not executing. Most trainers will complain that you should never have the execute light left on, so a surreptitious selection of 'Progress' satisfies all but the most pernickity.