This allows you to reserve a portion of your outgoing bandwidth for seeding and/or trading purposes.

Outgoing traffic is counted as trading when it is sent to another peer with the expectation that it will be reciprocated.  It is counted as seeding when it is sent to a peer that will not send anything back in return.

Note that these proportions are only enforced up to the point where it will not lower your total outgoing traffic.  If the portion you have reserved is unused, it will be used by any other remaining peers that need it.

More information is available on the Bandwidth View help page.