Boyz Mom, your vendor is confusing you a bit with a minor misuse of terminology ... and some of the answers above are continuing the confusion.
The Bottom Coil Tank you're looking at features a replaceable atomizer head. Replacement is dead simple --turn your tank upside down and unscrew the base from the tank. Then unscrew the head from inside the base. Pop in the new one, put everything back together and you're ready to vape.
The true "coil" is inside the atomizer head -- it's the bit of wire, wrapped around a wick, that heats up as it conducts electricity and vaporizes the juice pulled in by the wick. The coil is the part of the atomizer that eventually fails after repeated use, sort of like a light bulb burns out; it gets gunked up with residue and can no longer heat consistently or, perhaps, at all. You can tell a coil is failing when the taste gets nasty, the vapor production gets very low, or it simply won't "fire" at all. How long you can go between atomizer failures depends on your set-up, your juice, and your maintenance and vaping habits. Cleaning your atomizers routinely will extend their life and you can find lots of advice here in the forum on how to do that.
Rather than simply replacing atomizer heads, many advanced users actually rebuild the coil inside when it fails. They take the atomizer head itself apart to remove the old coil. Then they wrap new wire around new wick, carefully replace the old coil and check on a voltmeter that the resistance of the rebuilt coil is within the proper range. For the price of a few replacement heads, they can buy enough wire and wick to rebuild dozens of coils, experimenting and customizing to their heart's desire. It's fussy work, but many find it both satisfying and economical. If you're interested, the forum contains lots of how-to advice on rebuilding coils.
Personally, I love my bottom coil tanks. I find I get few dry hits and I've had no leaking problems. And I would rather soak my atomizers in vodka to clean them when they start to taste bad or vape poorly, then just replace them when they eventually fail altogether. Replacements run me $2-$3 each, and since I've only burned out about one a month since I've been using BCCs, rebuilding holds no attraction for me. YMMV.
ON PREVIEW: While I was drafting my opus, djtonyb and gthompson jumped in with excellent explanations. Thanks, guys. And the tank you're considering looks to me like an EVOD clone. Search the forum on EVOD for lots more info.