What is the difference between multicloud and non multicloud offers?

Multicloud means that you can add as many backend servers that are neither instances, nor Bare Metal cloud servers, nor Dedibox dedicated servers.
These can be services from other cloud platforms such as Amazon Web Services, Digital Ocean, Google Cloud, Microsoft Azure or OVH, but also on-premises servers hosted in a third-party datacenter.
Unlike the multi-cloud offers, non-multi-cloud offers allow you to add only backend servers part of the Scaleway ecosystems which include Instances, Bare Metal cloud servers, and Dedibox dedicated servers.