Im wondering how this Su-35"S" is any different from the two previous
Su-35BM that was tested last year. Which of them is really the final
The Su-35BM is the pre-production prototype... a test bed to put all the bits together in a real plane and see if they all work together. It is generally put together with available parts but not in a production line... it is hand made.
The Su-35S is the serial production aircraft, but by the article above it seems the S models were used to accelerate systems testing. There are lots of things that need to be checked like whether the new radar in the new nose configuration with the new IRST next to it is properly cooled and doesn't overheat the stuff next to it and cause problems. Stuff like that.
This article talks about the first serial production batch of Su-35S fighters that have been made in a production line rather than made as a single aircraft prototype. They have been tested and are being sent to the Russian Air Force who will send them to a testing and evaluation centre to draw up the various operations manuals which will be printed and distributed to the units when they get the new planes.
Serial production aircraft are made in a way that they can be put together in a factory production line. Changes will be made to the prototype design to make them easier to put together on a factory line so the serial aircraft will be different to the prototype so even if the prototype was fully flight tested, the serial production models still need flight testing to make sure changes made to make them easier and cheaper to make on a production line haven't effected performance and characteristics.
For instance on the prototype electronics might be put where it will fit for the moment, in the production model they might shift a few things around to make it easier to build and easier to maintain. Things might be shifted or access panels might be enlarged to make certain things easier to get at for example which might have nothing to do with how easy it is to make and put together. Or it might be found that two access panels close together can be combined into one which makes it quicker and easier to make and reduces the number of edges/panels/screws etc.