Most of the CEVs are armoured fighting vehicles that may be based on a tank chassis and have a turreted demolitions gun, but are not really intended for combat. Their specialties are entrenching, demolitions, and breaching obstacles. Other engineering vehicles are bridgelaying tanks, which replace the turret with a segmented hydraulic bridge. For example: bridge tank, dozer tank.
Another type of CEVs are armoured fighting vehicles which use as transportation to sappers (Combat engineer troops) and can be fitted with bulldozer's blade and other mine-breaching devices. They are often used as APCs because of their carrying ability and their heavy protection. For example: IDF Puma, Nagmachon, Huskey HVGP.
CEVs may also including civilian heavy equipement which was modified to fit for military applications. In that case - the heavy vehicle must have some sort of protection, usually armour plates and steel jackets. For example: the IDF Caterpillar D9 armoured bulldozer, American D7 TPK, cranes, graders, excavators French mine-breaching modification for an old version of the D9 etc.
Combat Engineering Vehicles' pictures from around the world (www.militaryphotos.net forums)