Virtual components are those which have a footprint on the PCB (and may additionally have a schematic symbol) but do not have an associated physical component which needs to be loaded onto the board during assembly.
Examples of virtual components include:
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Mounting holes
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Solder bridges
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Net ties
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Test points
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Fiducial markings
Virtual components must be indicated by setting the Placement type to Virtual
in the Footprint Properties dialog.