It is a sac
I do not do enough of this to know if there are differences in sacs apart from the size and the material. The sacs I used worked well for me and my pens are working again. I appreciated the packaging in that it was easy for me to understand which sacs were which, they did not just throw them all into the box and force me to figure it out. They always pay attention to detail. I bought a variety of sizes.
