The early MTS systems (MTS1 and the first MTS2 sets) used serial transmission of command pulses for things like sound and other functions - if you pressed function 1 (eg: for the whistle) it would send a single pulse, function 2 would send two pulses in succession and so on. This is why locos with older LGB digital sound systems have slow reactions to the higher function numbers - eg: if you press function 5, it takes a good couple of seconds for the system to transmit five pulses and for the loco to read and decode them before the correct sound is operated. With Parallel functions, which were introduced partway through the life of MTS2 and all of MTS3, the system sends a different signal for every function command, so the delay is avoided and each command is acted on almost instantly. The other advantage of parallel is that occasionally with the old serial system, one or more pulses could get "lost" or misread in transmission due to poor pickup, dirty track etc - so you could actually end up getting a different function that the one you wanted because the decoder miscounted the pulses; obviously with parallel this is avoided.
All modern parallel systems have the option to be switched back to serial mode for particular locos, eg: if you have one with an old-type sound system.
Edit: the fact that the change between serial and parallel occurred partway through the sales life of MTS2 is why you have to be especially careful when buying items of that vintage: if you're looking at used MTS2 kit, look for either a round black sticker with a white "P" or, with later ones, a "p" suffix on the part number (which must be on EVERY component of the setup - central station, remote(s), and wireless transmitter/receiver if used). MTS1 kit will always be serial only, MTS3 gear will always be parallel. All Massoth kit is parallel of course, but has the selectable serial function on an individual loco basis.
Jon.
