special ledger and parrlel ledger

hy,

sap gurus,

what is the difference between the spl ledger and parrlel ledger and which version is used in spl ledger and which version is used in parrlel ledger.

some my friends told me spl ledger is no way in 6.o version, its imp for 5.0 only, right 6.0 onwardsd parrlel ledger is important, is it correct

thanqu gurus

raja