table global action update field on row: all rows vs rows in context

What is the difference between a context of “all rows in model” and “rows in context” for an update field on row action in a table’s global action?

I want to update all the rows in the model, and I want to be able to use merge syntax in the context of each row.

HI Matt, do you happen to have an example laid out that would help illustrate the scenario you’re working through?