You should only use this feature if your datacenter has pre-defined MAC addresses for its IP ranges.
For example, OVH has a system that registers a specific MAC on per IP basis. This feature will allow you to specify the MAC for that IP in SolusVM.
Most datacenters don't check what IP is using what MAC. If your datacenter is one of these, leave the MAC field as auto.
If you assign a specific MAC to an IP, the system will detect the MAC on creation of a VPS. The system will check that no other VPS is using the MAC and create the VPS. If the MAC is already in use it will generate a new MAC for the VPS (You will need to edit it from the admincp).
MAC addresses are only used on creation of a VPS. If you assign one of the IP's as an extra IP, you may need to change the MAC of that IP in the datacenters system/control panel to match the MAC of the VPS.
In case you are working with KVM/Xen, then you need to have the following line in /usr/local/solusvm/data/config.ini file:
[KVM] ;; Add OVH style gateway routes to virtual servers (true|false) ovh_bridged_network_routes = true