What is an inner bath tub panel for?

I removed a Moldy wooden bath tub panel and right after it there is another panel which feels like hard foam.

Sorry I do not have pictures.

Do you have any idea what material this is and what purpose this panel serves? It is an old building I don’t think modern baths have these second panels.

What material is it and would it have to be discarded if it affected by mold?

Thanks.