In PLAXIS, all material datasets are stored in a single listable called Materials. This includes all soil material datasets, but also all material datasets for the structural datasets like plates, geogrids, beams and anchors.
With Python, when looping-over all material datasets, you might just want to get e.g. all plate material datasets. For this, we can use the property .TypeName to know what type of material it is.
For example, you can use this to create a list of just plate materials in PLAXIS 2D
platematerials = [mat for mat in g_i.Materials[:] if mat.TypeName.value == 'PlateMat']