I opened a Hornby 8 pin docoder today and one wire wasn't soldered at all and doesn't look like it ever was. So much for quality control at Hornby.
Just to add to Rog (RJ)s cryptic reply.
The Hornby R8249 [as do many other brands] has 9 wires connected on the decoder, but there is only 'as standard' an 8 way connector plug. This means that one wire [the purple / mauve one] has to be free and un-terminated as there is no connector pin spare for it. This wire is the fourth function wire and is switched negative by sending an F2 command from the controller. The wire can be used for any suitable use within the loco. For example a Cab Light.
The other three functions are Function 1 & Function 2 which are the 'Directional Lighting' functions controlled by F0 on the controller. The third function is the decoder 'Green' wire [Pin 3] on the 8 way connector and controlled by sending F1 from the controller.
Therefore it is not a 'Quality Control' issue. The decoder is designed that way and they are all the same.
Note: None of this information is applicable to the TTS decoder to which the original post relates. It is only applicable to four function 8 pin decoders such as the Hornby R8249.