The internal compartment is the physical equivalent of a secure datadrive. Usually concealed within the abdomen and torso, the internal compartment is a sealed, anhydrous environment designed to fool radar, ladar, tactile and other means of detection, stealthily rendering the compartment's contents entirely hidden.
Each internal compartment has a release catch somewhere on the augment's body - for years, you could tell an augment had an abdominal compartment by the quiet hiss emitted when depressing the zyphoid process - but this catch can be located anywhere, and requires a very thorough search to uncover.
Compartments have their own SIZ rating. It can be at most the bearer's SIZ -8. If an augment is large enough, then another individual can actually be stored within an internal compartment quite effectively.