According to the RoHS directive, a homogenous material has a uniform composition. Namely, it cannot be destroyed or divided into different materials with abrasive processes. For example, grinding, cutting, and crushing would not separate the material. Indeed, it would maintain its uniformity. Therefore, for RoHS, every material in a product needs to fall below the 0.1% threshold (or 0.01% for cadmium).
Whereas for REACH, an article would be any individual component. For example, a resistor would be considered an article. Thus, any SVHC above the 0.1% (w/w) threshold needs to be declared. Understanding this concept is a step towards RoHS and REACH compliance.