The Load Source option determines if the FEA is solving just the pole and cross arm deflection (Ruling Span) or is iteratively solving the pole and cross arm deflection along with the changes in conductor tension until and equilibrium is reached (Network Solve). 

Ruling Span

When the Load Source is set to Ruling Span the conductor tensions and sags are calculated using the ruling span method. These tensions are then applied to the structures which causes deflection. However with the Ruling Span option the deflection will not feed back into recalculating the conductor tensions. Hence the clearances and tensions calculated when using FEA - Ruling Span will be identical to the static analysis calculations. 

To illustrate this consider this example where the conductor is under high tension. 

Once FEA has been run with the Load Source set to Ruling Span the pole and cross arm deflects but the conductors stay in the original position. 

Note this will overestimate the deflection in this case as in reality as the pole deflects the conductor tension will lower hence reducing the overall deflection. 

Network Solve

When the Load Source is set to Network Solve the initial tension is calculated and then the pole deflection is calculated. This pole deflection is then fed back into the conductor tension calculations. This iterative process continues until an equilibrium is reached. 

Below is the same example as from the Ruling Span section however with the option set to Network Solve. Note it is showing the deflection is much smaller in this case because as the pole defects the tension in the conductor drops.