Refer to figure.
Traffic Load (TL) is is the total mass of passengers, baggage and cargo, including any ’non-revenue’ load.

Dry Operating Mass (DOM) is the total mass of the aeroplane ready for a specific type of operation excluding usable fuel and traffic load. The mass includes items such as crew and crew baggage, catering and removable passenger service equipment, potable water and lavatory chemicals, food and beverages.

Operating Mass (OM) is the DOM plus fuel, but without traffic load.

According to the figure, the Maximum Allowed Traffic Load is 15 500 kg, but the Actual Total Traffic Load is 13 500 kg.

Thus, the aircraft is underloaded by: 15 500 kg - 13 500 kg = 2 000 kg.

NOTE: Be careful to avoid any double mass calculations, due to the fact that some masses are already included in other masses, e.g. fuel mass is included in OM, crew and crew baggage mass is included in DOM, DOM is included in OM and so on.