I've always thought the best way to understand these types of licensing agreements is to treat it like you would a physical book - only one person (any person) can read the book at any given time, sure someone can look over your shoulder, but ultimately only one of you is driving (flipping the page when "your" done reading it). It's a simple personal use licensing, some products will have different licensing schemes for different types of use (and charge differing rates based upon the usage). i.e. commercial use licensing, educational licensing, ... but as far as I know for the majority of Sonar users there is one licensing scheme.
The enforcement (unless some form of physical check is performed say with something like iLok or a network licensing agent) is ultimately an honor based system. You "agree" to abide by the terms of the EULA and that's that.
As has already been stated an "upgrade" is not a new license, it's simply the same license for a newer version of the product, but the same basic rule applies. Sure you can run Sonar X1 on one system and Sonar X2 on another, but the honor code in place requires you to only use one instance at any one time. You can't gift it to someone else, you can't sell the old version as you really only have one non-transferable license, ... so on and so on.