I just upgraded to 5.3, and I noticed a field called, "line number" on the bottom right of the input screen. I can't find anything in the manuals on that (perhaps it's a new feature). I think I know how it works if it's similar to a poorly-implemented feature that was in another program I was briefly forced to use before I quit my last job. Could someone please point me to some documentation?

From the Main Menu, navigate to "\help\on-line documentation\User's Guide". When the PDF opens, search for "line number".

Note, you are correct, this is a new feature in Version 5.30. This is an initial implementation to see how the user base wanted to implement this idea. Since the release of 5.30 in November (2010), we've had a lot of suggestions. As a result, the line-number feature is currently undergoing a significant enhancement effort. If the line numbers aren't sufficient for your needs, look for improvements in "build #2" (we hope to release build #1 today, which won't have these improvements).