Influence length is a property of each Blade aerodynamical station. For each aerodynamical station lift, drag and moment intensity is calculated using BEM. Influence length is then used to calculate the corresponding lift force, drag force and moment.
For a FEM blade (i.e. flexible blade) these forces and moment are distributed and applied to nodes. For a stiff blade the forces and moment are used to caculate the thrust and torque on the hub node.
The influence length of an aerodynamical station is the sum of half the initial distances to each of the (two) neighbouring stations. The influence length for one particular blade aerodynamical station is illustrated in the image below:
The influence length NEVER changes during a simulation, the initial parameters are always used to calculate it.
Note: for the blade stations corresponding to the root and the tip of the blade, the influence length is the distance from the blade station itself to half the distance to the neighbouring station.
The influence length of an aerodynamical blade station can be found by right-clicking on the corresponding station, as shown in the figure below: