The categories game refers to the switching or misplacement of deviations into less competitive or algorithm preferred submission categories in order to gain a ranking advantage in dA's popularity algorithm, generally as a means of appearing more frequently on front page and subsequently gaining greater exposure.
The categories game arises from the inherent category biases under dA's "fair exposure" policy. The goal of the policy is to "maximize the display of the most popular deviations in a variety of topics
with Fair Exposure, the community is exposed to a varied cross-section of deviantART. " What this boils down to is that certain