Does the data usage meter measure all data usage or do I need to download multiple meters if I use multiple devices?
The data usage meter measures all data usage flowing through your cable modem. If you are using multiple computers and other devices behind your modem, such as on a home network, the meter will report data usage for all of those computers and devices combined. This also includes general DOCSIS-related IP traffic between the cable modem and the Cable Modem Termination System (CMTS) which is not possible for you to measure on your own.