From Halopedia, the Halo wiki
The warehouse is a Covenant structure used for collecting/processing resources. It can be built onto any unused Covenant or Neutral plot for 100 resources. It brings down supplies from orbit to Covenant bases via gravity lift. Workers move the supplies to the proper area for further use by the Covenant commander. At tech level 1 it can be upgraded to a Blessed warehouse for 225 resources to allow for faster collection of resources. It is the Covenant equivalent of the UNSC Supply Pad.
In order for a warehouse to be upgraded, a Temple must be built. The warehouse's resources are used to train units, construct buildings and research technology upgrades. It is ideal for the most efficient bases to be equipped with six warehouses to boost available resources. Of course, the number of warehouses you can build is limited to the number of base pads you have available. The best combinations of warehouses are five to seven normal warehouses, or three blessed and two normal warehouses.
The difference between the normal and "blessed" variant seems to be that the blessed variant is more advanced than the normal one, and looks more intricate, it has a larger sliding section and four more storage tanks. Obviously, this contributes to the increase of the resource gathering rate from 240 resources per minute (or 4 per second) to 330 resources per minute (or 5.5 per second).
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