PCL according to Microsoft :
Portable Class libraries can help you reduce the
time and costs of developing and testing the code. Use this
type of project to write and build .NET assemblies
Portable frameworks, and then reference those assemblies from
applications that point to multiple platforms like Windows and
In other words, projects like P ortable C lass L ibrary are functional when you want your library to be available for different platforms such as Windows phone and simultaneously for Xbox. Support platforms are the following:
While the Standard are those that are only supported by windows and .net framework. This means that you can not use in a windows phone or Xbox project.