That should say on the IQ chassis. On the IQ mounting your boards to the tunnel causes stress and eventually cracking. The lower part of the tunnel is not meant for support. There are a couple threads on this.
On other models, such as the REV it is perfectly acceptable to mount to the side. This is why a one size does not fit all sleds. Go with a company that designed their boards for YOUR sled. Not a company that designed their boards on another model and just made them to fit every other make/model.
