Ammunition in DayZ Standalone refers any item or collection of items that may be used with/in a weapon or firearm, this usually refers to cartridges, projectiles, explosives or anything used in similar nature that may cause damage or injury to one's self or others.
Firearms only accept certain calibers (or sizes) of ammunition. A firearms caliber is determined by the barrel diameter which the firearm is chambered in. Different types of ammunition can be found throughout DayZ, typically inside various buildings and commonly found inside military installations. They may be found as boxed or loose ammunition, or in loaded Magazines.
Individual rounds or shells each have their own item condition (badly damaged, damaged, worn, pristine) and when stacked together, the condition of the stack is essentially an average of those individual rounds. While most stacks found as loot spawns will be of uniform condition, rounds of any condition can be stacked together by players, and it is possible to sort the ammunition by splitting each stack down to individual rounds.