Questions about Hellvape GRIMM empty ink cartridge

Most of the Pod Cartridges in the market contain coils inside, but, do you know the empty Cartridge?

Why there is an empty Pod? From my perspective, it may simplify the replacement.

Let's look at today's topic—Hellvape GRIMM Empty Pod. 

The mouthpieces has black and white colors. It can contain 3ml juice. The airflow control is different from other Cartridge. Not only can it be adjustable, but also can switch from MTL to DL flavor. Sounds Great, right?

However, what kind of coils can the pod fit for? The answer is Hellvape GRIMM Coil Series! And you need to buy it separately.