The FEA Unstressed Lengths option controls how the unstressed cable lengths are derived:

The option Deflect Poles will hold the horizontal tension constant while the poles deflect; when equilibrium is reached, the unstressed lengths are used.

To illustrate consider the following example that has a single Mango conductor strung between two poles 50m apart with the stringing tension set to 17.7kN at 5oC. Neither pole has a stay on it (and hence the poles will deflect).

Using the option Deflect Poles the FEA will adjust the pole deflection and conductor length until the conductor tension is 17.7kN as can be seen below. In this simulation the derived unstressed conductor length was 42.49m which is less than the span length.

Using the option Rigid Poles the unstressed lengths for cables are calculated assuming their endpoints are completely fixed under stringing conditions. This approach is similar to assuming that all poles are rigidly guyed under stringing conditions, cables are tensioned to the specified tensions, and then subsequently the structures are allowed to deflect.

To illustrate consider the same example from above where there is a single Mango conductor strung between two poles 50m apart with the stringing tension set to 17.7kN at 5oC. In this case the starting tension in the conductor before pole deflection was 17.7kN, however with the pole deflection the tension dropped to 1.28kN.