According to Gutter Supply, a J-channel is a trim that holds the soffit panel in its rightful place. Riverside Sheet Metal mentions that J-channel is designed for roofing applications. The J-channel mounts to the structure of the wall.
The soffit is the underbelly of any construction piece.
Masonry screws are used to fasten J-channel to brick walls. Regular nails and screws are used for wooden structures. A standard measurement for J-channel is 12 feet and also measures 1-5/8 inches-by-1/2 inches-by-3/4 inches.
It is also made from 0.19-inch aluminum that is pre-painted. Riverside Sheet Metal mentions that J-channel normally comes in 10 inches, but it can be extended to 12 inches.