A fillet weld lays a triangular bead in the corner formed by two members meeting at 90 degrees. The 'leg length' (the size of each side of the triangle) is specified to match the load. Common gate fillets run 4 to 6 mm leg.
Fillet welds don't penetrate the parent metal. They sit on top of it. That makes them larger than equivalent butt welds at the same shear capacity: a 6 mm fillet is roughly equal to a 4 mm full-penetration butt. They are also easier and cheaper to weld because no edge preparation is needed.
CAD60 drawings denote fillet welds with the standard ISO 2553 fillet symbol (a triangle on the reference line) plus the leg size.