ANSWER
Straddle milling is a type of gang milling.
In gang milling, an assembled tool comprising two or more milling cutters mounted in the same arbor, machines several workpiece surfaces simultaneously.
In straddle milling, two or more side-and-face milling cutters, mounted in one arbor, machine parallel planes of a workpiece. The planes are perpendicular to the arbor axis and feature an exact distance (distances) between them. To ensure the necessary accuracy of the distance (distances), the milling cutters are spaced apart with the use of bushings and spacers.