Soap bar pickups are rectangular in shape and are typically mounted to the guitar with wood screws. The screw holes are recessed on the outer edge of the casing.  Fluence 7 & 8 string pickups with plastic finishes mount in this fashion.

Pictured: 8-string modern humbucker (black)

Tabbed humbucker pickups are rectangular in shape and can be covered or uncovered. These are usually surrounded with a pickup ring and or suspended from a pickguard. They have mounting ears that extend down below the lower plate of the pickup. The tabs extend out and the screws thread direct into the ears of the pickup.  All Fluence 6-string humbuckers are tabbed.  7-string humbuckers with metal finishes also feature tabs.

Pictured: 6-string Devin Townsend signature humbucker, 6-string open-core humbucker, 7-string modern humbucker (nickel)