A small assembly of standard or manufacturing parts that provides support Using Mylar to the panel is called a Support unit.
Mylar which is used as a support is called Support Mylar. One support unit can consist of one to multiple numbers of Support mylars. You can see a sample support unit in the given picture for reference.
In this unit, as you can see there are two support mylars. out of them, one is having one direction shimming and another is mounted with the help of an L block, which is providing two-way shimming to the mylar. Shape of the unit and parts depends upon the requirement of that tooling station.