So I've just discovered that whilst Administrators can use the Calculate function on a field (in a layer in a web map) to bulk fill a value through records, it won't apply it to only selected records but everything in that layer (very dangerous mistake to make - thankfully the field was empty to start with!). I've also just learnt that this function is only available to Administrators and not Publishers or Users. (Work with fields)
This seems counter productive to me. I know that calculations can be restricted by using Filters, but this defies the point of being able to make a quick selection and quick calculation by having to modify filters to change that selection each time (especially when there is already a filter in place for different purposes).
Can this be looked at as I don't really want to have to grant people Admin level just so they can use the Calculate function but it seems there is no other way around it at present.
This idea has been marked as a duplicate to Apply Calculations to Selected Records AGOL Calculator