In sex dolls, a removable vagina refers to a separate insert that can be taken out for cleaning or replacement. On the other hand, a built-in or fixed vagina is a permanent part of the doll's body, molded and attached directly to the doll.

The main advantage of a removable vagina is that it is easier to clean and maintain. The user can remove the insert and clean it separately, which is especially important for hygiene and prolonging the life of the doll. A removable vagina can also be replaced with a new one if it becomes damaged or worn out.

A built-in or fixed vagina, on the other hand, offers a more realistic experience as it is molded directly onto the doll's body. This can provide a better sensation and a more lifelike appearance. However, cleaning and maintaining a built-in or fixed vagina can be more difficult as it requires cleaning with special tools and solutions to prevent damage to the doll.

In summary, a removable vagina is easier to clean and maintain, while a built-in or fixed vagina provides a more realistic experience. The choice between the two depends on personal preference and how much importance is placed on realism versus ease of maintenance.