Has anyone heard of this concept?

We have something in my unit that I assume is what you are describing. My ICU has a separate 6-bed section next to the main ICU that we use as ICU during times of high census, and operate as an Intermediate ICU/PCU when census is lower. It's staffed by ICU nurses usually so that they can become critical care beds as needed.

We recently implemented Adaptable Acuity beds in our hospital. These are on the med-surg unit and staffed by nurses with additional training. These nurses also have a reduced patient load. Our Adaptable Acuity beds are used for patients that don't quite need CCU, but need more attention/monitoring than our tele beds.

so how do they schedule staff for this? have people on call for when acuity increases?

We staff with our med-surg nurses (around half of them are trained), although sometimes it means our capacity isn't as high. The unit we place them on usually goes up to 20 patients. If for instance, we have 2 Adaptable Acuity patients (and no extra staff) then the unit can only take 18 patients. Otherwise, the AA patient can go to CCU if they aren't full, or we will call in a nurse if one is available, and house them on the unit.