What is Parent Content Type in SharePoint 2010?As you know content type in SharePoint gives re-usability features.
Parent Content Type is a hierarchical relationship between two content types. Parent Content Type defines the properties for a child content type which inherits all properties of parent content type.
For Eg Lets create a document library called "Sales Report". Now create a content type called "Sales Documents" which contains few site columns as well. You may apply "Sales Documents" content type to newly created document library called "Sales Report". By adding this content type "Sales Report" document library will inherit all properties from its parent content type.
So a question may arise here that, why we need parent content type?
Answer - Suppose you have applied this content type "Sales Document" to more than 10 document library. Now if you got an requirement to add a new site column to all 10 document library. so in this condition you need not to add site column one by one to each and every document library. Instead you just add a site column to parent content type, now all content type inheriting this parent content type will add this site column to all document library.
One more important thing is once you create a content type, you can not change its parent.