The second role of guerrilla bands is to attack and capture French supply convoys.
As the French advance they rely more and more on supplies forwarded from their main base. They can move up to four days supplies (each sufficient to supply one corps for one day) between depots each day. Supplies move at the same rate as a corps on the march.
If a supply convoy enters a map square adjacent to a guerrilla band the guerrilla may attack. The rules for such attacks are discussed in the last of this series of blogs here:
http://napoleonicwargaming.blogspot.com/