Until today base material was automatically detected based on the diffuse map. However it was often inaccurate and could only store one reference. It is now required to manually enter the base material references. For 95% of the parts there will be only one value, but for elements like the dual moulded legs or the Batman armour 34706d1 you may need to add additional references. values shall be separated by a comma and the first one represents the master value.
This value is currently only used for rendering. It shall be a valid JSON array. You shall not define it yourself but only copy and adapt it from a similar existing part.
Whenever roughness values are not standard, they shall be specified for each base materials. Two parameters are available: strength and size.