The dominant and submissive tags are used extremely broadly, often simply being used as synonyms for "topping" and "bottoming". The wiki page for dominant is quite broad, but does conclude with what I think is a good rule of thumb:
A general test for domination/submission is whether there is an imbalance of power, whether it be real (held at spear point) or permitted (holding a leash).
However, even this can be broad and subjective. Technically, all adult_on_young, teacher/student, unprofessional_behavior, etc. relationships can be said to feature an "imbalance of power"; where do we draw the line? It's also often tagged with size_difference, even in images with no restraints or obvious power imbalance, such as post #5450930.
Getting every user to read the wiki and obey the definitions is probably a hopeless battle, but I wonder if we could at least revise the wiki pages to lay out clearer definitions and boundaries?