The marker list function in the NA mode of the 4395A/4396B displays "EXTRA" value in the list below Smith chart. What does this value stand for?

It stands for equivalent capacitance or inductance. It is calculated from the reactance value, which is indicated as AUX value, by using the following equations:

Capacitive reactance (when the AUX value is negative): X_C = 1 / (2 x pi x f x C)

Inductive reactance (when the AUX value is positive): X_L = 2 x pi x f x L

Where, f is test frequency, C is capacitance, L is inductance.