What is the difference between the -f and -l options of secure-delete srm? - Ask Ubuntu

regarding secure-delete command in linux, difference between srm -f, srm -l , srm -fl ? fastest? secure?

-f fast (and insecure mode): no /dev/urandom, no synchronize mode.

-l lessens security. 2 passes written: 1 mode 0xff , final mode random values.^{ubuntu manuals}