Almost all large-caliber weapons in Spring: 1944 (20mm and larger) have a system of different ammunition types that the weapon (or rather, the unit armed with the weapon) can switch between.
By far the two most common types are High Explosive and Armour Piercing ammunition types, available to almost every size cannon of 20mm or larger. Other ammunition types exist as well, such as the "buckshot" shell of the US 37mm gun and the flak/shrapnel shells of various anti-aircraft guns.
Most units have more than one type of ammunition, and this is part of what gives these units their roles. For instance, all battle tanks have both High Explosive and Armour Piercing ammunition, making them highly adaptable (the US M5 Stuart has High Explosive, Armour Piercing and Buckshot ammunition types, the most of any unit). Other units, such as Tank Destroyers or Artillery, have only one type -- Armour Piercing for tank destroyers and High Explosive for artillery -- which makes them more "specialized" in certain roles and weaker in others.
How a unit decides between which ammunition type to use is completely automatic and requires no input from the player -- a tank will automatically fire High Explosive ammunition at infantry, vehicles, structures and other soft (unarmoured or lightly armoured) targets, and will switch to Armour Piercing when the need arises to fire at armoured targets. How each ammunition type acts can also play a part. This mainly has to due with //Armour Penetration// that is covered elsewhere, but basically, units will choose whatever ammunition type best suits what they're firing at, even taking into account the dynamic range-based properties of armour piercing shells (and how they lose their penetrating power the further they travel).
In-game, the different ammunition types are represented with visual differences.
High Explosive shells, when fired, will be a constant solid yellow colour, and will generally explode on impact with the ground or their target.
Armour Piercing shells start out a deep red colour that will slowly turn yellow the further they travel as they slow and weaken from air friction. When they land, they produce a shower of sparks as they splinter into fragmented metal shards.
File:Heap.jpg round on the left, HE on the right}} //Above: The tank on the left fires a glowing red Armour Piercing round, while the tank on the right fires a yellow High Explosive round.//
The "buckshot" ammunition fired by the US 37mm is quite identifiable as a shotgun-like spread of small rounds.