Anything that the other enemy brought with them that could be carried away, in addition to many things that belonged to the locals surrounding the army. It is another ugly detail to not the most shining example of human times.

There were two main sources of pillage and loot, the fallen and captive, and the baggage train. Usually the most valuable would be wealthy captives that can be ransomed or diplomatically used, in addition they often had the most valuable arms and armor. For the nobility, there were often different customs and norms, like being treated better and perhaps even being allowed to keep some items. Even more humble captives might have something hidden or some ability to pay a tiny ransom. Then of course there was the fallen, whom the poorer soldiers would likely be carrying everything they brought with them that they owned (which wouldn't be a whole lot). Care for the dead sometimes proved difficult. Only the nobility could adequately rely on being interred with Christian rites. Anyway, what would be looted would be coinage, precious metals, jewelry, weapons, armor, clothing, horses, and most importantly food.

The baggage train would often have any non-combatants (camp followers, relatives of soldiers), perhaps some treasury, food supplies, pack animals, livestock, and campaigning gear. Depending on the size and wealth of the army this could be a significant boon. In fact, the outcome of battles have changed due to the temptation to attack/loot baggage trains prematurely.

There was also a precedence in the spoils and loot. Often the most choice items and lions share would be awarded or taken by those of higher social standing (which is why soldiers might try to hide what they looted, or attempt to loot with the confusion of battle still ongoing). Common soldiers took advantage of the chaos to stash what they could in looting; meanwhile, the upper classes preferred the spoils to be distributed. That way those in command could see it distributed as they wanted (to themselves, mostly). In short, they could be at odds.

I can't actually say how much a single soldier could loot, mostly because the more attributed occasions cite what the nobility received. However, it could be whatever they got their hands on that they were not by way of law, custom, or force made to hand over. It was however in the interests of the elites to make sure their soldiers received some plunder.

Anything left on the field and not buried or hidden (soldiers sometimes hid valuables nearby before battle) is likely to be looted by exactly what the name implies - looters. People that follow the armies or from nearby who search among the battlefield for anything of value.

Campaigning armies were not known to be kind to areas they marched through, even to their own countrysides. When writers say foraging and living off of the land, that often includes purchasing, requisitioning, or even stealing supplies from locals. More prepared rulers would often set aside grain stores specifically for their marching armies, as was done in Roman times. Maintaining discipline was always a challenge. Unchecked marauders often caused diplomatic disputes, such as a vassal subordinates levies looting an ally of their liege.

R.H.C. Davis, A History of Medieval Europe

The Hundred Years War: The English in France 1337–1453 Desmond Seward

Medieval Mercenaries: The Business of War