DPart metadata is associated with pages or page ranges. It was specified in PDF/VT first and is defined in PDF 2.0 as well. it is intended to be used in automation to allow for processing pages in the same PDF in different ways. This can be done in pdfToolbox using QuickCheck which is explained here...

But pdfToolbox also is as of today the only tool that can easily display such metadata.

When a PDF has DPart metadata a button is indicating this at the bottom of the pdfToolbox window.

A suite of sample files including the one below is available from the PDF Association:
https://www.pdfa.org/resource/cal-poly-pdfvt-test-suite/

Clicking on this button opens the DPart viewer.

It shows Records, leafs and DPM nodes. Records are expected to be present in each DPart hierarchy on at least one level to define where records are differentiated. pdfToolbox shows right after the name (in this example "Record" the pages with which the respective record is associated.

DPM nodes contain the actual metadata which may in the PDF use any format (key - value pairs, XML, PDF syntax) but is in pdfToolbox converted into JSON.

Leafs hold the page associations which may be page ranges, but are in this example only single pages. The leaf entries contain links to the respective pages in the DPart viewer.

An Export button allows for saving the structure as a JSON file to the disk.