RFringe is “relative fringe”.
In emacs, displaying fringe indicators is done via text overlays. In that way, bitmaps in the fringe are attached to the lines of text shown in the buffer window.
This works nicely when the fringe is used to indicate information that is relevant to the adjacent line; for example, text overflow, or something similar. But, there isn’t a simple way for an application or module to use the fringe to display buffer-relative information - for example, the location of compiler error messages.
In fact, using the fringe to communicate this kind of information - buffer-relative positions - is probably more intuitive and is certainly more useful for the user.
rfringe.el addresses that.
rfringe.el includes an extension to flymake, to display the buffer-relative positions of error messages in the fringe. This is not the only purpose of rfringe.el. It could also be used to display relative positions of diffs in diff-mode, for example. It could be used to indicate document sections. It could be used to indicate positions of “TODO” or “FIXME” items in a document. Use your imagination for other possibilities.
Here’s an example of rfringe showing indicators in the fringe for flymake errors, in a vb.net buffer:
indicators.el provides similar functionality but offers many improvements, like per-pixel resolution and ability to add indicators whose positions are determined by markers or functions. It also provides minor mode to quickly control the display of indicators.