In MC 15 and earlier one could create a global definition using tilde ~ instead of :=.
The resulting expression had the "three bar" equal sign instead of the colon equal sign.
I found that quite useful for creating new units of measure at the bottom of my document.
For example, I frequently use Giga Joules so I would define GJ equal to 10^9xJ.
I don't want to clutter up the top of document with this type of definition and the tilde global definition allowed me to put these definitions at the bottom of the document.
But MC Prime does not seem to have this (to me) useful capability.
The global definition operator is planned for Mathcad Prime 3.0.