Level Five Skills Needed
From Backstage Lore Wiki
To emphasize how long training is needed to gain access to specific item, L5SN is the number of different skills you need to train up to level 5.
Example: Sin needs skills Gallente Battleship, Spaceship Command, Electronics, Jump Drive Operation, Navigation, Warp Drive Operation and Science all trained up to level five thus resulting to L5SN of 7.