I hope the title is self explanatory. I was unable to find anything about the term on google however my intuition says that an "inline" tester would intercept some signals going from A to B and then display some information about the signal.
This is how I would visualize an "inline" testing device in my head:
A --------> [Testing device] --------> B
So maybe it's the language barrier or maybe I'm getting scammed, but what I have been visualising is not something that is supported by my newly purchased "Inline PoE Tester". Before I decide how to proceed I wanted to make sure that I got my terminology right and the word "Inline" doesn't mean something completely different in this context.