The e-liquid in the tank must be covered. Just be sure to check that the mod and the tank, if purchased separately, are compatible. Subohmic tanks offer the best flavor and bigger clouds than any other atomizer accessory for a vaping device, and are the most popular accessories for vaping devices in the world today. Some subohm tanks are a little thirstier than others, but when vaping below ohms, in general, you'll notice that the juice goes away much faster than when using a mouth-to-lung vaping tank.
When vaping in a public space, it's always good practice to consider others when exhaling steam, but it's even more important when vaping below ohms, as the amount of vapor created is much greater than that of a regular pen vaporizer. Subohmic tanks are usually designed for use with advanced vaping kits, such as box mods and larger vaping pens. Subohm vaping is an important part of the vaping world today. Just a few years ago, manufacturers of homemade coils had access to this style of vaping, but since then the number of options for subohmic tanks has been increasing, making it easily accessible to everyone.
For example, if you're a beginner to the world of vaping and don't have experience with rebuildable atomizers, mechanical modifications, or battery safety, you can safely say that you're not ready to vape below ohms. Battery life is always going to be an issue on smaller devices (and pen-type devices), especially if you vape a lot. Subohm tanks are popular with experienced vapers because they have a number of unique advantages that differentiate them from other types of vaping accessories, such as above ohm tanks and vaping capsules.