The phrase "to reuse the shell" means using something again instead of throwing it away. It is often used metaphorically to talk about saving and recycling resources, ideas, or solutions instead of creating something new. Full definition
Care Instructions: Knock solids into toilet (reuse shell until soiled or wash day). (diaperjunction.com)
These systems have shells (or covers) with different soakers and you change the soaker but reuse the shell unless it's soiled. (the-cloth-diaper-connection.com)
And because the insert is against the baby's skin, if it's just wet, you can remove the insert and put in a fresh one, while reusing the shell — that allows you to stretch your diaper stash a little further between washes. (mommyedition.com)