Most of this information is directly from the Sekonic L-308 manual. I have only included parts relevent to the OP & condensed it. I assume that he doesn't have the manual.
The L-308B has two flash metering modes. One with just a Lightning Bolt & the other with a Lightning Bolt + C. "Lightning Bolt" mode is wireless, "Lightning Bolt + C" mode is wired.
The "Lightning Bolt" mode senses when a flash/strobe fires and gives you an EV (exposure value) or f-stop reading. The "Lightning Bolt + C" mode is used when tethered to the L-308B to tell the meter when the flash/strobe fires.
The L-308B also measures in 1/10th's of a stop. The 1/10th's are not shown in the numerical readout, but rather are shown on a LCD bar dial in the lower center of the screen (same LCD bar dial that indicates battery condition "b.c" when powering up the meter).
Each bar that is illuminated on the dial indicates 1/10th of a stop. The 1/10th bars are added to the numerical reading to give you the total value. For example, a numerical reading of f/2.0 plus five bars equals a total reading of f/2.4 (+5/10th's is halfway between full stops of f/2.0 & f/2.8). 3/10th's is about 1/3rd of a stop, 7/10th's is about 2/3rd's of a stop.
The ISO & Shutter Speed need to be set in the meter before taking a reading. If the measuring range of the meter is exceeded, the digital f-stop number is replaced with "E.u" (underexposed) or "E.o" (overexposed).
To tether to the L-308B, you would use a two prong PC cord of some type. Exactly which cord to use would be determined by what gear you're using & how it's configured. The three prong camera port is for a remote shutter release.
