After contacting support, this is the answer they gave:
Aggregate outbound bandwidth, in megabits per second, across all attached network interfaces (PublicNet, ServiceNet, and Cloud Networks). Outbound public Internet bandwidth can be up to 40% of the aggregate limit. Host networking is redundant, and bandwidth is delivered over two separate bonded interfaces, each able to carry 50% of the aggregate limit. We recommend using multiple Layer 4 connections to maximize throughput. Inbound traffic is not limited.
Picking an example: first row 512MB standard instance, RXTX factor 80.00/mbs is the maximum speed across all the network interfaces (public, private, cloud network) that data can leave the server
The first paragraph of this can be found in their docs
This seems like a bit of a twisting of what RXTX factor intuitively should mean and is not a ratio but actually in Mb/s.