The FME reader detects the version of the source dataset (version 7 or 8) internally and handles it accordingly. There is no difference to users in terms of the reader keyword or attribute names of the elements.
The IGDS reader first reads the header information from the Design file being processed, and extracts the conversion parameters required to translate coordinates from internal IGDS UORs to ground units. These units are all relative to working coordinates. See ASSUME_MATCHING_UNITS for further details on DGN V8 reading. It also determines the dimension of the input file.
It then extracts each individual element, one at a time, and passes it on to the rest of the FME for processing. Complex elements are extracted as single FME features. If a complex element contains an arc, then the reader automatically converts it to a linestring enabling it to be processed by all other readers and writers within the FME. If the element had any attribute linkages attached to it, these are read and added as attributes to the FME feature being created.
When the IGDS reader encounters an element type it does not know how to process, it simply ignores it and moves on to read the next element.
DGN Version 8 also reads the models to which the features belong. All the models read retain their respective working units and global origin values.