One of the advantages of the new SL system is the ability to use also Leica T and M, S, R plus Cine lenses with an adapter:
The Leica SL and the Leica T share the same lens mount, now named the Leica L-Mount. The Leica T and future Leica TL lenses are compatible with the Leica SL camera. The SL changes the sensor format automatically to APS-C size. Leica SL lenses can be used in the future on Leica T cameras. Currently, with the Leica M-Adapter T, there are 48 M lenses which can be used on the Leica SL. With the Leica S-Adapter L, there are 16 S lenses which can be used on the Leica SL while 51 R lenses can be used on the Leica SL with the Leica R-Adapter L. There are 21 Cine lenses which can be used on the Leica SL with the Leica PL-Adapter L. The different adapters make the Leica SL camera one of the most versatile cameras available to a professional photographer.
In addition to adapters, there are also several other Leica SL system accessories like multifunction handgrip, flash, filters, straps, batteries and chargers. Some of these accessories will be available from November 2015 while the others will be available from June 2016. The prices on some of the SL accessories are already listed at B&H: