Absolutely nothing.
It is simply a label, it has no technical meaning other than the tape and drive, must be the same 'density', and the rule is that for a tape to be allowed to be put into a drive the density must match.
So, an hcart2 tape will ONLY go into an hcart2 drive.
If the drive is LTO1, or LTO5, or 4mm, or DLT it makes no difference. A drive can be any density.
It is easier to think of them as colours :
eg. hcart = blue
hcart2 = green
4mm = pink
hcart3 = black
.. then a green tape (eg hcart 2) which physically can be any type of tape, will only go into a green drive.
There is one exception, never use qcart density (1/4 inch) - this is 'special' as it writes in a fixed block size, as this was a limitation of that type of drive. You don't see these anymore so it is pretty much redundant.
Unfortunately, there are not enough hcart densities to go around as we have LTO1, LTO2, LTO3 etc ... so if you run out, just pick one of the others (apart from qcart), it (despite people trying to argue it with me in the past) will not affect tape capacity or performance.
Unfortunately, despite it appearing to be a simple concept, the 'density' is very well embedded in the NBU code so getting these densities changed or added to is not going to be happeneing any time soon.
