Parallel 6th means there are two ported chambers with the sub mounted on a baffle between them. The two ported chambers don't interact with one another, but work together to create the passband. One is tuned high, one is tuned low. Those two frequencies determine the passband. In this case the high side is at 83Hz and the low side is 17 1/2Hz. That's the enclosure in the first picture.
A series 6th has two ported chambers, but one chamber plays into the other and there is one port for air to move in/out of the enclosure. Since they work together, changing tuning on one also changes tuning on the other.
The parallel 6th has more efficiency over the sealed enclosure I had in the thing.
An IB "enclosure" isn't an enclosure at all. You're just separating the front wave created by the driver from the back wave. In this instance the front wave will be horn-loaded off the wall I'm going to use to separate the front wave from the back wave. I'll be able to do normal SQ listening with it like always, but I'll open the hatch for SPL type stuff. For the sake of the hatch.