I am trying to score a measure which states that .... the total work exposure score is "the standardized mean of the subscales A, B & C". Does this simply mean the total score = the sum of the three subscales divided by 3? Or does it refer to the "standardized mean difference"?

The three subscales measure different things (A measures frequency of exposure to various events, range 0-635; B measures range of exposure using dichotomous items scored 0-1; & C measured intensity of exposure using dichotomous items scored 0-1)

I've searched on google for "standardized mean" but it only comes up with it in relation to the "standardized mean difference". I was wondering if the authors simply mean to refer to the mean of the subscales? Any advice would be really appreciated, am trying to write up my thesis so the pressure is on! Many thanks.