Reason behind the naming convention for the non-gate terminals of JFETs?
For an n channel J-fet it is the voltage between the gate and the lowest non-gate terminal which controls how hard the J-fet is turned on with the J-fet fully turned on when the gate voltage is equal to the voltage at the lowest non-gate terminal. Most J-fets are symmetrical and so, for a n channel j-fet, the lowest voltage non-gate terminal is defined as the source.
For an n channel J-fet the gate must be taken negative with respect to the lowest non-gate terminal to turn the J-fet off (depletion device) and the gate must not be taken much above the voltage at the lowest non-gate terminal or it will forward bias the gate diode.
Everything is reversed for P channel J-fets. It is the voltage between the gate and the highest non-gate terminal which controls how hard the J-fet is turned on with the J-fet fully turned on when the gate voltage is equal to the voltage at the highest non-gate terminal. Most J-fets are symmetrical and so, for a p-type J-fet, the highest voltage non-gate terminal is defined as the source.
For a p channel J-fet the gate must be taken positive with respect to the highest voltage non-gate terminal to turn the J-fet off and the gate must not be taken much below the voltage at the highest non-gate terminal or it will forward bias the gate diode.
The definition of the source as the lowest voltage non-gate terminal can cause distortion problems in ac applications such as wien oscillator gain control where the gate is held at  a constant negative voltage with respect to one grounded non-gate terminal and the other non-gate terminal is oscillating about ground. This second oscillating, non-gate terminal will be the drain on positive swings and the source on negative swings and so the J-fet will be turned on more during negative swings (the gate voltage is closer to that of the source, the lowest voltage non-gate terminal) causing output distortion. A couple of high value resistors around the J-fet efficiently circumvents this problem.