A Rally Point is a location, which can be set for various buildings. It indicates the target location, where newly created as well as ejected garrisoned units will move to. Rally points can be set for cities, the Market, the Senate, most of the military structures and the Terra Cotta Army wonder. A building's rally point is indicated as a flag/standard, when the building is selected.
A rally point can be cleared (reset to default) by using the shortcut
Shift + R, while the building is selected. New units will then again appear next to the building.
Cities have a number of rally point present for Citizens, which can be used to send Citizens directly to gather locations within the cities economic radius. It can be manually set on a construction site (e.g. of a wonder), to directly send new citizens to that site. A rally point of any production building can be set on a garrisonable structure (like a city or Fort), newly created units are then automatically send to that target structure and will garrison it, if possible. The rally point of a market can be used to direct new Caravans to a city, from where they will create new trading routes, and to direct Merchants to rare resources, so they will deploy automatically at the resource. Setting a rally point also affects, on which side of the building the unit leaves it.
Using units as Rally Points Edit
Setting a rally point on a unit not only sends newly created and evacuated units to that target unit's position, but they will also automatically join the target unit's group, if it has one. Note, that created/evacuated units will actually move to the target unit's position, not to the unit itself. So moving the target unit to a different location won't send created/evacuated units currently on their way automatically to the new position.
The Airbase's rally point is special in that it is used to direct all of it's aircraft to a particular target location. If it is set somewhere in the world, all aircraft from the base (whether landed or somewhere on a mission) will immediately be sent to that location and, once there, attack enemies in it's vicinity (bombers will attack only buildings by default, while fighters go for units). If an aircraft runs out of fuel, it returns to it's base, reloads and then flys to the rally point again (unless the Repeat Orders option of the airbase is disabled).