This is the theoretical maximum data rate this VDSL line could sustain right now given its current SNR and attenuation conditions, which by definition will always be equal to or greater than whatever rate is actually configured and running on the line. It's the number that tells you how much headroom exists to increase a customer's provisioned speed without the line actually being able to support it, useful when evaluating an upsell to a higher service tier. A line configured to run at 20 Mbps but showing an attainable rate of 45 Mbps has significant unused capacity, whereas one showing an attainable rate barely above its configured rate is running close to its physical limit.