In the layout which has mosaic instance, there are layer call "ref" which is defined as 0:0 stream number and datatype. It is in tsmc technology.
Could you please let me know which exactly it is?

I have check creating mosaic instance. If instance in the same library with current layout, there is no ref layer. If instance in different library, there is ref layer.

In this thread, @erikl has explained about it, but I'm still confused about "background" definition