Looking at the guidelines in the wikis for cum_on_upper_body and cum_on_lower_body, there is no consistency between them and the more established upper_body (and subsequently, lower_body) tags.
The upper_body tag has existed since almost the beginning of Danbooru (though back then it was known as bust) and the wiki states that "upper body" is considered as "approximately from the navel up", while lower_body's wiki states that "lower body" is considered as "approximately from the waist down". Contrastingly, cum_on_upper_body states that it is for cum "on the upper body (arms, chest, or head) of a character" and cum_on_lower_body is for "cum present anywhere on the body below the chest". Both of these "cum on x body" tags have different guidelines than the upper and lower body tags - the split between upper_body/lower_body is at the navel, while the split between cum_on_upper_body/cum_on_lower_body is between the chest and the stomach. This disconnect is probably a big part of the reason why these cum tags have no meaningful distinction.
I can see people arguing that the stomach is part of the upper body, but I can also see people arguing that it is part of the lower body. As such I don't think it should be considered as either because people will never reach an agreement on the subject, which means cum_on_stomach should be something of a no-man's land - not cum_on_upper_body, but also not cum_on_lower_body. This does not exactly mirror the guidelines for the upper_body and lower_body tags, but since they are different kinds of tags (image composition vs object tagging) doing so would serve no purpose. What it does accomplish is standardising the general measurements for the terms "upper body" and "lower body".
As for which tag cum on hands/arms should get, I'm of the opinion that anything above the elbow should be cum_on_upper_body while anything below it shouldn't receive either tag, as they rest near the hotly-contested stomach area. Even if we decide it can potentially count as both "cum on x body" tags because the forearm and hands can go both above and below one's navel, this still means that it cannot be aliased to only of them. At the very least, the alias for cum_on_hands need to be removed.