A Watch Commander, also known as "Field Marshall", is an intermediary rank within the Black Hand. He is the leader of the Black Hand operations in a given territory.
Watch Commanders are generally (but not always) the senior Hand member in a given city or region. Often they hold the rank of Dominion, though sometimes not, depending on the territory involved. Watch Commanders for major Sabbat cities are almost always dominions, as are those assigned to build up Black Hand strength in areas bordering Camarilla-dominated territories.
Commanders are appointed by the Seraphim for their strong leadership ability and administrative skill, but they often demonstrate political sensitivity, though they are not necessarily the known emissaries for the territory they oversee.
It's much like the role of a Ductus, however, fills a more permanent (or at least, long-term) position with similar responsibilities - to oversee and lead a territorial cell of all Black Hand members residing in that area. The Commander keeps an eye on local Sabbat politics, and ensures that all Black Hand members partake of the Vaulderie with each other, in addition to the ritus with their own packs. He is the de facto superior officer for all members under her Watch not assigned to a kamut.
He also serves as the official conduit for communications from the Hand leadership down to the local members, including updates to the codes in current use, watchtower level, and news about other Black Hand actions (as appropriate); he may be a teacher of Black Hand philosophy and catechism as well.
He may recommend a promising candidate for initiation to the dominions (even if he is not a dominion himself); report on local political affairs to the Seraphim's emissaries ; or provide recommendations to dominions looking to fill a kamut as to the skills and qualifications of members of her Watch.
If asked to command a kamut himself, he is responsible for appointing a lieutenant to command the Watch in his place (though as a rule, Commanders are not asked to lead kamuts, given their responsibilities locally) .
Caine's Chosen: the Black Hand, p. 28-29