Many material properties depend on two objects. For example, the friction of ice on rubber is different from rubber on rubber. The material property combiner computes the coefficient that is actually used in the simulation to simulate the physical behavior at a contact.

Combining material coefficients can be done in different ways. The accurate way would be to look up the exact value in a material table. The simple way is to use a mathematical operation to combine the coefficients, e.g. multiply both coefficients or compute the average. For most scenarios in games, the latter method is sufficient.

Reference