Worcester Greenstar combi boiler hot water settings: What does 'e' on the dial mean?

‘e’ for economy (or efficiency) perhaps?

Condensing boilers are at their most efficient at around 50-55 C water output temperature.
So going to a lower temp can be less efficient, just as going higher can be.

Personally, like many people, I like my hot water (and radiators) to be a bit higher than that at around 60 for the tap and 65 for the radiators

This is a good video about it although it is more about radiators than the tap water (because the radiators are usually on for longer than a tap): https://www.youtube.com/watch?v=T022zY9I__0&ab_channel=TheHeatingHub

Or just try it at the tap and adjust it to what feels OK once the tap has been running for 30 seconds or more. (Careful not to scald yourself, start cooler and work up).

Remember that for a mixer tap or a shower then how cold the cold water is (how cold it is outside the house) will make a difference to the temperature after mixing.