This just depends on terminology. For me, a "belly roll" involves only the abs - the spane stays out of it. With an undulation or body wave you bring in the spine and also change the angle of the pelvis.
But, you can then layer a belly roll over an undulation. Which is great - if that is what you want to do. If you always do the two together because you cannot separate them, then I'd say there was a problem.
