Following the most recent developer diary that introduced the changes in the Officer Corps, in this one, a new feature is being added, called Spirits.
Spirits are a new way to specialize your army, generals, and help design your divisions. Each military branch has several categories of its own spirits that affect different aspects of that branch's work.
Spirits have direct combat applications. Not merely used as buffing tools, Spirits give players the ease to use historical tactics during battles, which were hard to pull off in earlier patches. Additionally, parts of a division may be customized using Spirits, to best suit player's strategies in warfare.
Aside from that, academy Spirits, a special variant of the feature, are meant to be used in conjunction with the promotional mechanic mentioned in the previous instance of the developer diary. With these, commanders are no longer generated by chance but can be customized in advance, along with other needed personnel, from the beginning.
However, players must keep in mind that politics and ideology come into play when taking into account which kind of Spirit is available to the player.
The final feature is the preferred tactic. Using this feature gives players a larger chance of said tactic being used during a battle, making a favorable outcome more plausible. Using this feature alongside the Spirit customization of division and units seems to be the way to go, as their effectiveness is doubled. It would appear that unified tactics across all aspects of your military are the most optimized option, as well as a natural way of making players follow historical accuracies.
What all these changes show is that RNG is going to play less and less of a role in Hearts of Iron 4, which, in a strategy game, is always a good thing.