The meaning of scalable bandwidth is once the allocated bandwidth to your account is used in that case more bandwidth will be assigned to your account by taking extra charges for that.
Lets suppose you have a vps hosting width configuration
disk space: 40GB
bandwidth allocated per month(data transfer): 2TB
Ram: 2GB
Uplink Speed: 100Mbps
scalable bandwidth: 1TB/ 20$
Now meaning of this scalable bandwidth is : once you used 2TB less then a month in that case more 1TB will be assigned to your account by taking extra money 20$ per 1TB.
and that’s called the scalablity charges.
So if a hosting provider told you that they have scalable bandwidth in that you can ask them the charges of bandwidth scaliablity.
other terms also exist like CPU Scaliablity