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The precursor to XBlock. This class is responsible for generating a student-facing HTML rendering of course content. It is bound to a particular student on instantiation.
XModuleDescriptor
DescriptorSystem
This class provides an interface between XModuleDescriptors and the rest of edx-platform. The intention is to abstract the XModuleDescriptor away from the details about how external systems are implemented.
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This is used to mean an XBlock that uses none of the doesn't inherit from XModule or XModuleDescriptor methods, and access accesses none of the DescriptorSystem or ModuleSystem interfaces to access edx-platform.