Remnant port of the AreaOnAreaOverlayer FME transformer does only exists on the 2018 (or newer) version of FME. However, I only have access the 2017 version for now. Do you know a way to round it by using others transformers or method which would returns the same output ?
OK, so as far as I can tell, the remnants port outputs leftover pieces of geometry from the overlapping process.
Generally, if two polygons overlap, then both the overlapping and non-overlapping areas are output as polygons, so neither of them are remnants. Non-polygon input is output through the Rejected port, so that's not a remnant either. And neither is a polygon that doesn't overlap anything.
So what is a remnant? Well, if the tolerance setting is greater than the size of that overlap (say you had a 1m overlap and the tolerance was set to 2m), then the overlap is collapsed down to a line.
That line is a remnant.
But this depends on the tolerance of the process and in your older version of FME there is no tolerance parameter. Therefore no features would be reduced to a line and there should be no remnants.
In short (as far as I can see) the remnant port was added to support the tolerance parameter. Without a tolerance parameter there is no need for a remnant port.
So you won't get remnants in 2017, but you may get very tiny slivers in the output that the tolerance value would handle in newer FMEs. To resolve those slivers there are a few different ways. You could use the SliverRemover (since renamed to the AreaGapAndOverlapCleaner) to automatically clean them up. Alternatively you might use an AreaCalculator/Tester combination to locate slivers (features with a tiny area) and flag them for manual editing.
Another solution would be to clean the data before the it is overlapped. You could try the Snapper transformer in Vertex Snapping mode to do this. Basically it gives you the ability to set a tolerance to clean the data, which is (more or less) what the tolerance setting in the Overlayer now does.
But if your data is clean in the first place, you won't get any slivers and so the remnants port would be irrelevant.