Interpretation of Isolation in SPDT Relay

What exactly "isolation is -30 dB " means in SPDT Relay...and how is the signal on one line (whic is not closed) is going to affect the signal on the other (which is closed path...)

Isolation is specified in dB of rejection, according to either one of the following formulas:

Isolation (dB) = 10 log ( P_OUT / P_SOURCE )
or
Isolation (dB) = 20 log ( V_OUT / V_SOURCE )

and -30dB means that the signal on the other side of the contacts will be at least 30dB below the signal present on the primay side ..

IanP
:D

In the above diagram What are the effects on the signal of NC line (if NC is carrying the signal) due to signal on NO line (NO is also carrying the signal)