Type 1 (41) style cutting wheels have a flat hub and are generally used for straight cutting applications.
The flat hub design allows for more of the wheel to be used during a cut with less possibility of interference with the hub. Type 1 (41) style cutting wheels are commonly used when cutting profiles, corners and c-channel.
Type 27 (42) style cutting wheels have a raised center hub making it ideal for getting into tight corners, overhangs or where extra clearance is needed. The raised center hub allows for the locking nut to be recessed providing more of a continuous cut over the type 1 cutting wheel.