My Invisible Zipper or Concealed Zipper comes in 3 weights: #2, #4, #6. Some manufacturers mark their sizes as #3 & #5. They are a bit smaller in the nylon coil diameter but usually my pulls (also called zipper heads) will fit their coil.
2mm: Best in light weight fabrics, dresses that aren't very tight.
4mm: Use in medium weight fabrics for pillows, duvet covers, garments.
6mm: This is heavy duty for use in tight garments (wedding gowns), pillows in heavy upholstery fabrics, slipcovers.
Invisible zipper by the yard is less expensive than a finished zipper & more easy to use because you only stock yardage & pulls, not all the different lengths of zippers. You may purchase "with pulls on" the tape which means I put the zipper heads on the tape for you or loose which means YOU put the zipper heads on the tape as you use the yardage.
I have been testing this weight of invisible zippers in cushions and in the backs of slipcovers for many years. I'm pretty impressed but there are still the limitations that you have with all invisible zippers. They don't take as much stress as a standard coil zipper. To take the stress off the zipper when closing, I make sure that I pull the zipper tape together prior to trying to zip it up. And yes, this means it takes 2 people to zip a bride into her dress!
I've tried this in cushions up to a 4" box. These zippers are reliable up to 2" foam & wrapped with dacron, but 4" foam with dacron or down wrap is too much pressure on the zipper.
If you wrap your cushions with dacron, just make sure that you don't zip any dacron into your invisible zipper. It will pop & you can't repair it like a standard coil #5.