B-Line Eaton series imperial cable tray and ladder, 6" height, 91.11" length, 42" width, Series 2-5 horizontal bend, 90°, 12" radius, Aluminum

That is exactly what it is sized for. The 6A-42-90HB12 is a Series 2-5 horizontal bend with a 42 inch width and 6 inch side rail height, in aluminum, so it mates to a straight aluminum tray run of the same 42 inch inside width and 6 inch loading depth. If your run is a different width or a 3, 4 or 5 inch rail height, this bend is not the matching fitting.

Plan for roughly 91.11 inches of overall length on the fitting, which is the dimension B-Line publishes for the 6A-42-90HB12. On a 42 inch wide tray turning 90 degrees at a 12 inch radius, the outer sweep is what eats the space, so check the corner clearance to walls, ductwork and beams before you set your trapeze hangers.

A horizontal bend like the 6A-42-90HB12 turns one run through a single change of direction in the flat plane, 90 degrees in this case. If the run has to branch, so cable continues straight and also breaks off to the side, you need a horizontal tee or cross instead. And if the tray has to change elevation rather than direction, that is a vertical inside or outside bend, not this part.

Order by Eaton B-Line model 6A-42-90HB12. The number reads out as the spec: 6 inch aluminum side rail, 42 inch width, 90 degree horizontal bend, 12 inch radius. Splice plates and hardware are ordered separately from the fitting.