JLPCB Assembly board cleaning extra charge?

[rcbuck]

I am going to have JLPCB assemble 20 boards for me. I see under Advanced Options, there is an extra charge to clean the boards after assembly.

Are the boards not cleaned after assembly as a matter of procedure? Or is this charge for some kind of special circumstance?

If you have had them assemble boards for you, were they clean and free of solder flux when you received the boards?

I had a chat conversation with them and they said the boards have to be evaluated after assembly. I have no idea what that means.