What is a via metal type?

Geometry Elements - Metal Types

A via metal type specifies the metalization loss for via polygons. Each via polygon in your circuit is assigned a via metal type (lossless by default) and you may have as many via metal types in a project as you wish.  Since the metalization loss is unrelated to the location of the vias, changing the metal type of a via does not change the physical location of the via. It changes only the metalization loss.

Via metal is used only to model via polygons; planar  metal types are used to specify metalization loss for planar polygons.

Each via metal type can be represented by a different cell fill pattern in the project editor, chosen when you define the metal type.

For more information on via metals and defining loss for via polygons, please see the "Via Metalization Loss" section of the "Metalization and Dielectric Layer Loss" chapter in the Sonnet User's Guide.