The first change implemented in Civilization 6 is the concept of support units, wherein something like a siege tower, battering ram, or anti-tank or anti-aircraft gun can stack with a typical military unit.
Second, the concept of grouping into formations. Calling it a long-requested feature, Shirk says you’ll be able to link two units (one military and one civilian) together and will be able to move them around together with a single movement command. “That’s really nice with civilian units, as you can escort settlers or builders or missionaries around the map.”
Finally, to minimize late-game unit bloat, Firaxis has created Corps and Armies: two or three of the same unit type (a Rifleman, for instance) combined into a single, more powerful version of that same unit. A Rifleman Corps unit would be roughly 40% more powerful than an individual, which means they’re less potent than the two component Riflemen would be by themselves, but together they’re more survivable and potent in a single attack. Having the option to create Corps (unlocked in the Napoleonic era), and then later add a third unit to create an Army (unlocked around 100 years later), is designed to open up new tactics and reduce overcrowding on the map.