All a mat zone is, to use Poser Vernacular, are a set of facets that have been grouped by the usemtl tag in the obj file. In this case the material, or usemtl, is grouped under the tag usemtl Borgebody.

If you import an object into Poser that has no usemtl tags Poser will assign one called Preview, in a similar fashion to files with no Vertex Normal, or vn information.

One usemtl. Or, if you prefer, one "mat zone"

https://people.cs.clemson.edu/~dhouse/courses/405/docs/brief-obj-file-format.html

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