What is the second, smaller compartment in my dishwasher's soap holder for?

In the place where you put the soap into my dishwasher, there are two compartments, one larger and one smaller. They both get closed in with the same lid after you put in the soap. The part of the lid that covers the smaller compartment has 3 narrow slots in it. What is that smaller one for? I always just put soap in both, but I’m not sure if I should.