A Transport Bay is any internal space aboard a military or civilian vehicle devoted to short- or long-term human habitation or cargo. Generically known as "bays", these storage spaces are used to carry vehicles, personnel or bulk cargo aboard larger vehicles or spacecraft. While almost all unit types allocate some amount of internal space to transport bays of one sort or another, BattleMechs and ProtoMechs are the exception to this rule, with the effort of incorporating transport bays into their design seen as a waste of weight. Despite this, there are rumors that some forces, especially Clan Hell's Horses, have attempted to create so-called "walking APCs" though this is pure speculation.
 Cargo Bay
Cargo Bays are found on nearly every type of unit from Combat Vehicles to DropShips and larger spacecraft. Most are simply open space within the unit's framework, with just clamps and other anchor points for retention straps, while more specialized variants include fixed-cubicle style "container" bays, refrigerated, insulated, liquid and even livestock storage bays.
Container Cargo Bays weigh 10 tons and take up one space slot/provide 10 tons of carrying capacity per container. All other Cargo Bays have a variable weight and either take up one space slot per ton for 'Mechs or one total on vehicles; their capacity is dependent on the type of cargo they are designed to carry.
 Infantry Bay/Compartment
Infantry Bays are typically found on large vehicles and vessels such as DropShips. They provide basic sleeping arrangements, a communal waste facility, and in some cases stowage compartments for food and equipment. Life support is either drawn in from the outside environment through air filters and conditioners or the ship-wide life support system.
Infantry Compartments differ in that they are meant to transport infantry over short distances and are typically found on smaller vehicles. Beyond space needed to carry soldiers and their gear to where they are going, compartments lack any of the amenities found in the larger bays.
All Infantry Bays and Compartments takes up either one space slot on vehicles or one slot per ton on 'Mechs. Each Bay/Compartment has a capacity of 28 troopers except for Mechanized Infantry (which carries 5 troopers) and Battle Armor (which carries 1). Foot Infantry Bays/Compartments weigh 5/3 tons; Jump Infantry Bays/Compartments weigh 6/4 tons; Motorized Infantry Bays/Compartments weigh 7/6 tons; Mechanized Infantry Bays/Compartments weigh 8/5 tons; Battle Armor Bays/Compartments weigh 2/1 tons. All Infantry Bays/Compartments can be combined into a single, unified infantry bay slot, regardless of tonnage or number of troopers, when used on combat or support vehicles.
 BattleMech/Fighter Bay
BattleMech/Fighter Bays consist of one or more cubicles for transporting 'Mechs, Aerospace or Conventional Aircraft of up to 100 tons over long distances. Each component cubicle in these bays includes the maintenance and launch facilities for a single 'Mech or craft, along with basic living accommodations for the unit's crew and technical support personnel. These bays are commonly found on DropShips or very large support vehicles.
Each 'Mech and Fighter Bay takes up one space slot, weighs 150 tons, and has a capacity of one 'Mech or craft of up to 100 tons per Cubicle.
 ProtoMech Bay
ProtoMech Bays take up one space slot, weigh 50 tons and have a capacity of 5 ProtoMechs per 5 Cubicles.
 Small Craft Bay
Small Craft Bays are designed for the long-range transport of a single Small Craft of up to 200 tons. Like similar transport bays they provide landing, launch, maintenance and accommodations for a Small Craft, its crew and support personnel.
Small Craft Bays take up one space slot, weigh 200 tons and have a capacity of 1 Small Craft of up to 200 tons per cubicle.
 Vehicle Bay
Vehicle Bays are provide the same basic facilities as 'Mech and Fighter bays, though they lack the same orbital launch, drop or recovery systems. Each cubicle is configurable for different sizes of vehicles, though they fall into the range classes of Light, Heavy and Super-Heavy. In a pinch a Super-Heavy Vehicle Bay can carry Small Craft.
All Vehicle Bays take up one space slot and have a weight comparable to their maximum carry capacity weight. Light bays may carry a vehicle of up to 50 tons, Heavy bays up to 100 tons, and Super-Heavy bays up to 200 tons.