Surface Marshal was the title borne by the commander of Imperial Army forces within a Sector Army. Along with the Navy title of High Admiral, this title was typically borne by the Moff or Grand Moff, though in some Sectors, it was delegated to a senior subordinate.
While all Moffs and Grand Moffs had viceregal authority over the Imperial military forces within their Sectors, there is no evidence that they held any specifically military commission additional to the titles of High Admiral and/or Surface Marshal; it seems likely, therefore, that the rank insignia worn by Imperial governors were in fact indicative of these military ranks. It is not clear, however, how much "Surface Marshal" was a formal grade in the Army chain of command, and how much it was in itself a form of state dignity.
The title of Surface Marshal was introduced in The Imperial Sourcebook; the same publication shows a rank plaque for a Priority Sector High Commander, which corresponds to the badge worn by Grand Moff Wilhuff Tarkin in Star Wars Episode IV: A New Hope, except for a lack of code cylinders. This may have been the insignia of the Surface Marshal/High Admiral of an Oversector; for a fuller discussion of the badges shown in various canon sources, see the Rank Insignia of the Galactic Empire page.