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In Project Settings > Locations, you have the option to set the Spatial Type to 'Linear' when editing Project Location Levels.

With the jobsite selected, click on the 'edit' button to edit the selected project Level.  In the Add Level popup,  Add a Level Name and select a Spatial Type:  Map - Polygon.

In the 'Edit Jobsite' panel, click on the 'spatial Type' to view options.

Select the 'Linear' option under 'Spatial Type'.

With the 'Linear' Spatial Type selected, click the 'Unit' bar to view and select one of the Unit options.

With 'Spatial Type' and 'Unit' selected,  click the 'Save' Button to save your selections.  You have now set the Spatial Type to Linear.