All servers within a Project can talk to each other via private RFC1918 address space (e.g. 10.x.x.x), but cannot communicate over private address space with devices outside of that Project. This is also referred to as "backend" networking.
This feature ensures that when you're communicating between servers, you are able to do so in a private / secure manner without needing to worry about establishing VPN tunnels or sending data over the public internet.
The only restriction is that all servers must be within a single project.
Backend transfer within a facility is always free, since no bandwidth leaves the datacenter. When transferring data our global facilities, we have to move that traffic across our network. As such, bandwidth is billed on a usage basis at a reduced rate of $0.03/GB. There are no monthly or other fees associated with Global Backend Transfer - just the cost of sending data between facilities.
Enabling Backend Transfer
In the client portal, browse to the IPs & Networks page, and then the Backend Transfer tab, for the project you wish to enable backend transfer on, and then click the toggle icon in the upper right-hand corner.
Backend transfer will be enabled on your project immediately, although you may have to wait up to 1 minute for backend connectivity to be established.