The Apex planet forms the peak of a triangle shape with two other planets in some aspect patterns. It is usually a focus or outlet for the combined energy of the whole aspect configuration.
This often happens in symmetrical shapes: the T-square or Yod or Arrowhead. It can also appear in asymmetrical patterns like the Wedge, Cradle and other triangle-shaped aspect configurations.
The apex planet provides a focus or outlet or release or an extra dimension for the other two planets. For example, in the case of a T-square, the apex planet squares the other 2 planets that are in opposition.The Apex planet may act as the dominant energy in a 3-way argument between the figure's planets.
The apex planet needs to distinguish itself through its position. Equilateral shapes would not have an apex planet; for example the Grand Trine or the Grand Square.