From Halopedia, the Halo wiki
The crates come in two different sizes, large and small, and are typically green and grey in color, with white light emanating from within the crate itself. The crates are marked with letter-number designations and markers, such as "UNSC EXT [0044 KC]". The crate-like devices performs as part of a slipspace-shielded containment area, upon which a translocation beacon is launched.
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