The configuration of a catenary system in a span, which determines dropper lengths and spacing, as defined by dropper tables for each design catalogue.
The below image shows the span type selection option in Lineardraft, located in the 'Span Types' tab in the 'Edit Wire' window.
The second image shows the same for an individual wire point, located in the 'Edit Wire' dialog.
More info: Find out how droppers are calculated.