Polygon Collection features correspond to element type 106, which was added in Bentley Map Select Series 3.
These features represent a logical grouping of area elements. In XFM datasets, they can represent the shared Bentley Map XML-based Feature Modeling (XFM) data of the grouped elements. They are part of the XFM data representation as a normalized representation of XFM features. See the Bentley Map XFM Design format for an alternative object representation.
When these elements are associated with XFM data, they may contain XML XAttributes representing XFM feature properties. They may also contain a set of XFM XAttributes which consist of Universally Unique Identifiers (UUIDs). These XFM XAttributes may occur on both XFM Polygon Collection elements and on other graphical elements in order to associate these elements with each other.
XFM features may actually be represented by a hierarchy of elements related by XFM XAttributes, and it is the relationship of an element with XFM properties, such as a polygon collection, and an associated element that may be a part of this combination of graphics and properties. The focus of the DGNV8 Reader is to read XFM feature stores for the purpose of providing XFM properties with a specific graphical element in an XFM feature hierarchy.
There are no attributes specific to polygon collections. Refer to the XFM XAttributes section for XFM-related attributes (which they may have).