Difference between Datum and Datum Plane in Layer properties

[jesterphat]

I went thru a couple of simple exercises adn could not make out the difference... anyone actually know?

 

[rjmcgill60]

It may have something to do with datum planes set as gtol references. I've noticed that when adding datum planes to layers, if it is set as a gtol reference I have to use the datum plane type identifier but if it is not set, then the feature type identifier works.