Belt requirement for the protected cruiser?

[scaffold]

Hello everyone!
I believe this has been already discussed before, although I tried using search but found nothing.
Why ship design application has a requirement for the protected cruiser to have a belt? AFAIK, the essence of protected cruiser is that sloped deck stands in for the belt armour and it has no belt per se. Still the game requires a belt to be put on a protected cruiser and it costs weght.

[aeson]

Within the game, the 'belt' armor of a protected cruiser represents the sloping part of the armor deck.

[xenontennessee]

Brilliant question. There must be a little problem in the discription.