Hi Jason,
No worries. This area of GIS has been getting more, rather than less, complicated over the years!
A realization is sometimes called a re-adjustment. All the control points in the network are run through a least-squares adjustment (which shakes everything up a little) and then the control point coordinates are updated based up where they are at the time. It takes into account subsidence, tectonic motion, seismic events, etc. Sometimes there are transformations (which are just best fit) between realizations, sometimes not, or the transformations are released years after the realization is put into practice. Pretty much every 'new' geographic coordinate system (datum) that was based on ITRF/WGS84 uses the same ellipsoid, GRS80, but how that surface fits the earth will vary a bit.
ITRFxx and WGS84 (Gxxxx) have more mathematical definitions and are tectonic plate-agnostic. They vary very little over time. NAD83 and its realizations/re-adjustments are bound to the plates (North American, Pacific for Hawaii and American Samoa, and Mariana for the Guam and the Marianas) to try to minimize the change in coordinates over time. That works well for a lot of places, but the west coast, Louisiana, Alaska, Hawaii all have issues. That also means that the NAD83 realizations are moving away from the ITRFxx/WGS84 (Gxxxx) frames. NAD83 <> / != WGS84 any more!
The county systems like MN and OR got particular realizations versions depending on when they were added. The WisCRS system was added when the HARN re-adjustments will still going on in some parts of the US, but Wisconsin had had their done for a while. We added a NAD83 (2011) set at 10.4.1. The Oregon sets were added in discussion and cooperation of the Oregon DOT. Basically, it's very difficult to add and maintain several versions of the projected coordinate systems based on different realizations so both we (ESRI) and the EPSG (an online registry of coordinate reference systems and transformations) try to limit them.
Melita
