Perennials that fall into this category of partial sun(or shade), are plants that only require 4 or more hours of sun. An eastern or western exposure will provide the required number of hours of light for this category. Another example could be a southern exposure that is partially blocked by trees, buildings, etc.  Again, like full sun perennials it is important to assess the number of hours of direct sun you have where you want to plant. If you live in the southern part of the US, you probably should adhere to the 4 hours, some plants will not perform well because of the heat and the intensity of the sun. Many gardening mistakes can be avoided by thinking about where you are planting and what goes on in that site, is it dry, or too wet, how hot is the sun, etc.