Most probably the image attached to this post will explain everything, but please let me tell you a little bit about it.
Thanks to that a basic infantry division can be ultimately promoted to the enlarged armored unit (corps in this case). Achieving that needs a lot of time and lot of Production Points, but for an experienced unit, the result might be worth to give it a try.
Imagine that you have got an infantry unit that participated in many battles during early campaigns and received only little losses. This makes the unit very well experienced and allows it to be even more efficient during future battles. However with the time passing by, infantry unit becomes too slow and this reduces possibility to engage enemy. Wise decision would be upgrading this unit to motorized division. This will increase speed of the unit and will allow to catch many more enemies. Next step will be upgrading the unit to armoured division. This will boost strength of the unit and make it much more deadly for enemies. Last and ultimate step is upgrading this unit to armoured corps. This will upgrade strength of the unit even more and with the proper commander make it a very efficient part of the spearhead.

Please feel free to ask questions about this (and others) feature and let me know what do you think about that.