Every 'Mech supports three types of internal cockpit items: Standing, Hanging and Mounted. These are all cosmetic only; they do not give any in-game advantage or enhancement. However, some provide XP or C-Bill bonuses or increased Faction Reputation. Some are simply holiday decorations.
Each item has a unique position within the cockpit, which may change depending on which 'Mech chassis you are placing it in. The Cockpit item Warehouse is filtered by Standing, Handing and Mounted, and can be further filtered on "All", "Owned" and "Sale". Many include sounds and movement - select the "preview" button to see the animation or hear the sound.
Cup of coffee, mug of beer or a bottle of the Inner Sphere's finest bubbly (2395 - a very good year). Some bobble with movement, some are continuously animated, some jump out of a trash can with every kill!