The MultiLib substrate is build by a stack of layers.
You can stack different materials.
Every layer has a property such as material-layer,conductor-layer,gound-layer,via-layer,...
All layer types come in different versions, one loss-less version at least and mostly one or more lossy versions. For details have a look at the specific layer-documentation.
The substrate is defined by a data item ixSubst and some layer data items, which will be described in detail in capter 2.2 in this manual.