EMC's mod policies revolve around ensuring that each player on EMC has an equal advantage in gameplay. Mods such as X-Ray, speed hacks, etc. are not allowed because they allow a significant advantage over the other players. This is encapsulated by the rule above, as found in-depth on the Rules and Moderation page. These types of mods are often found on the Disallowed Mods list, but even if some do not, they are not permissible (see the important statement below).
Approved Mods may include mods such as Shaders and Inventory Tweaks or other mods that have no effect on physical game abilities and are focused on player aesthetics. These are also all client side, which means they only affect the player and don't require the server to have any mods or plugins installed to run.
IMPORTANT: If you do not find your desired mod on the Approved Mods list, and it is not a single-player mod with no function on EMC (see Incompatible Mods), consider it DISALLOWED until your request is approved.