Preferred unit type

From EU3 Wiki

You can choose one unit you prefer in each of three categories: infantry, cavalry and artillery. Your core provinces will produce your preferred units. Also, all the units already built will be automatically changed to the new preferred one. Their morale will be set to zero though so make sure you aren't in the middle of a war while doing this. You can't upgrade your mercenaries and those available to you in provinces are usually backward to what you can produce.

More units are available to choose after you reach higher levels of Land tech. You can't choose ships, they are assigned automatically. You choose your preferred units using the Domestic Military screen.

Usually the choice is between better offensive or defensive morale or between higher and lower rates of shock and fire damage.

Also, the type of units you are able to choose from depends on your research group and culture. For example only Germans can choose Landsknechten Infantry and only muslim countries can produce Muslim Cavalry.