The mod implements milk as a fluid so it can be used in piping systems and storage devices.
Both buckets and bottles have been registered as milk storage containers and can be filled/drained by appropriate fluid related mechanisms. However, due to default Minecraft behavior, neither bottles or buckets are capable of placing milk into the world, although an empty bucket can retrieve already placed milk from the world. The milk bottle can be drunk, similar to the milk bucket, and will remove negative status effects.