What is immersion and envelopment?

Immersion and envelopment relate to how much a patient sinks into a support surface (immersion) and how much the support surface comes up around the body (envelopment).  The idea is to try to maximise the potential body contact with the support surface so that the total surface area is increased to reduce the interface pressure. The larger the surface area, the more pressure can be redistributed to lower the interface pressure.