LSL & SSL Stand for Long Swing Link and Short Swing Link. These are not acronyms as such (unlike talking in station codes for example) but are actually to do with the design and type of bogie underneath the Mk3s.
I could sum it up but this cold is taking too much out of my tonight, so see post No.9 on this forum about it. If you look closely at the picture of the Bogies, you can just about see where the LSL & SSL come into play on the bogie and why it's unfortunately such an issue - see under the 'spring plank' / central air suspension.
http://www.firstgreatwestern.info/coffeeshop/index.php?topic=15602.0
In short though, SSL bogies are permitted to run onto the Southern region. LSL Bogies are not, due to the risk of the swing links coming into contact with the third rail.