What is the purpose of of the gray slider bar in the Fill Color layer icon?

[KlausKi]

In the Layers panel, I noticed that all Fill Color layer icons have a gray slider bar illustration at their bottom:

What is the purpose of that slider bar icon visualization? What is it inducing?

[Semaphoric · Accepted solution]

This is also true for Gradient an Pattern Fill Layers. Looks like  it's to indicate an editable Fill layer rather than a pixel layer.

[KlausKi]

Ah, interesting … Thanks a lot for enlightening me!