I am analyzing a JFET. The datasheet has both information about the currents Igss and the maximum of Ig. I know that the Igss is a the reverse current in the PN junction but I can't explain myself what kind of current is Ig and when does it flow?
The maximum value (Ig) is when operating (or misoperating more precisely) the gate of an N channel JFET with a positive gate voltage with respect to source - Vgs of an N channel JFET should normally reside between 0V and some negative value (like -10V). Between these values is where the JFET controls the drain current/load.
It's basically like an old triode tube/valve - you took the gate negative to control anode current.
Putting ancient history/fokelore to one side, if you have Vgs at 0V you have a fully conducting channel i.e. maximum drain current and, if you took Vgs more positive, you'll find there's a parasitic diode that starts conducting hence, the Ig limit is 10mA but, whilever the gate is held at 0V or less, the gate current is in the order of nano amps.