Storage __ (4)
(Other definitions for retention that I've seen before include "Enter into custody" , "memory" , "holding" , "Keeping hold (of something)" , "Failure to eliminate a substance from the body" .)
(Other definitions for unit that I've seen before include "Body of soldiers" , "hospital department" , "Single thing or person" , "Operational team" , "Group that's part of a larger organisation" .)