The load capacitance CL is a factor for determining the "conditions"of a crystal unit when used in the oscillation circuit .In an ordingay oscillation circuit the crystal unit is used in a range where it functions as an inductive reactance in such usage ,the oscillation circuit operates as a capacitive reactance .
In other words ,when the oscillation circuit is seen from both terminals of the crystal unit, this oscillation circuit can be expressed as a series circuit of a negative resistance -R and a capacitance CL . At that time this capacitance is called the load capacitance .The relationship between load capacitance and oscillation frequency is not linear.
When the load capacitance is small, the amount of frequency variation is large ,and when the load capacitance is increased, frequency variation lowers .If the load capacitance is lessened in the oscillation circuit to secure a large allowance for the oscillation frequency ,the frequency stability will be greatly influenced even by a small change in the circuit. The load capacitance can be chosen from standard values specified in the catalog. but 10-30pF is better.