For each PDF/A-1 validation issue we try to explain what it means and, if possible how to resolve it. For a more technical explanation of the issue we quote the related section of the ISO 19005-1 PDF/A Specification.
The definition of block-level structuring elements should follow the strongly structured paradigm as described in PDF Reference 9.7.4.
All non-standard structure types shall be mapped to the nearest functionally equivalent standard type, as defined in PDF Reference 9.7.4, in the role map dictionary of the structure tree root. This mapping may be indirect; within the role map a non-standard type can map directly to another non-standard type, but eventually the mapping must terminate at a standard type.