Specifies the maximum throughput of this link, in bits per second.
This is used to calculate the percentage utilisation, which in turn is used to make the colour for the link arrow, and optionally the label on the link.
The second form allows you to have 'asymmetric' links, like an ADSL, where the first number is the maximum bandwidth from node1 to node2 and the second is the maximum from node2 to node1, as they are given in the NODES line.
Bandwidths can also use K,M,G and T suffixes to specify large values. Also see the KILO global option though.