Since last week, .be domains can be protected against outgoing transfers by placing a transfer lock, just like you’re used to for .com domains and other gTLDs. This lock is an additional protection for the domain holder, next to the authorization code.
Please note that the registry does not provide authorization codes if a domain is locked. If one of your customers wants to transfer his domain to another provider, first remove the lock and then request the authorization code. The same applies for transfers-in of course: the current registrar will have to remove the lock before you can initiate a transfer.