Your iSub G Tank is a clearomizer or glassomizer that can only utilize disposable heads however, some of the head types can be rebuilt.
A Rebuildable Tank Atomizer is very similar in appearance but has a build deck section for you to replace the coil and wick.
There are some tanks that can be both like the
Kanger Subtank Mini and if you follow that link you can see the photos of it's disposable heads along with it's RBA section that turns it into an RTA when used.