Basically courtyards can be categorized into two major type.
1) A space, which is cover by other independent buildings and has a common access through the courtyard. One of the most commonly seen examples is a college building or a university campus
2) A space, which is within a single building, and various spaces can be access trough the courtyard. One of the most commonly seen examples for this type is a home design with courtyard in the center or a hospital building.
For both these types the criteria for designing landscape is quite different because the difference in the nature of human activities. There are many more factors that govern the courtyard landscaping designs. Some of them are
1) Size or volume of the courtyard (measured in length, width and height)
2) Whether it is open to sky or not
3) The amount of sunlight it receives
4) Does it have any cross ventilation