Storage Building Modes

[upper copper]

Accept

All beavers will add and remove items as they are needed. "Accept" warehouses do not generate hauling tasks.
This is the best setting for your bulk storage.

Supply

Haulers will attempt to move the contents of the warehouse to any warehouse accepting that resource, and will only fill them if every other warehouse is full. "Supply" warehouses do not count toward the percent fill setting on district crossings.
They are best used as output buffers (e.g. log pile near lumberjacks, plank pile near a lumbermill).
Non-haulers ignore this setting, and treat the warehouse as normal.

Obtain

Haulers will attempt to fill the warehouse from other warehouses, and will never remove stock from them.
They are best used as input buffers (e.g. log pile near a lumbermill). Putting warehouses set to "obtain" near construction sites to minimize how far builders need to haul materials can often double or even triple how quickly large projects can get built.
Non-haulers ignore this setting, and treat the warehouse as normal.

Empty

Every available beaver will help to remove stock from the warehouse. This should only be used sparingly, as it can cause issues with other jobs.

Storage Building Modes

[fresh palm]

Note that obtain warehouses for input buffers work best when medium size or larger; this allows haulers to have the spare capacity at destination to pick up larger packages and have them in transit.

[fresh palm]

Tangentially related (touched on with "non-haulers ignore this setting") - Beaver transport behavior

• When emptying buildings (flags, factories, crossing, things set to empty, etc) beavers will take the goods to the nearest storage, as long as it is not set to empty. This means that a warehouse next to a bunch of factories set to empty will be the destination of their outputs.
• Supply mode actively pushes to the nearest 'accept' or 'obtain' with available space
• Obtain will actively pull from the nearest non-obtain warehouse