What is the difference between a fixed and a removable vagina?

Sex dolls are available with a fixed and a removable vagina. From the outside, both vaginas are identical. The difference is on the inside.
With a removable vagina, there is a sleeve in the vagina that you can take out to clean. Nowadays, that sleeve is so firmly stuck in the doll that you no longer feel the difference between a fixed and removable vagina. The advantage of a removable vagina is that you can easily clean it.