Bottom bracket spindle length
Axle length and spindle length are the same thing.
Three-piece cranks typically specify a required axle/spindle length to achieve a specific chain line, I.e. how far away the chain rings are from the center plane of the frame.
An axle/spindle longer or shorter by x millimeters will move the cranks x/2 millimeters outboard or inboard respectively.
Some manufacturers are very good about specifying axle/spindle length for individual crank models. Some provide a recommended bottom bracket model from which the length can be found, others provide no information at all.