Grammar

Apologies for the minutiae in advance. :) Never heard this use of "they're". I couldn't find a source here but as a native speaker, I think if you have a clause ending in "they're" you remove the conjunction and just make it "they are". 
It seemed unnatural so I'm proposing this to avoid confusion.

Author: JustinGaravel

articles/active-directory/develop/app-objects-and-service-principals.md

@@ -17,7 +17,7 @@ ms.reviewer: sureshja
 
 # Application and service principal objects in Azure Active Directory
 
-This article describes application registration, application objects, and service principals in Azure Active Directory (Azure AD): what they're, how they're used, and how they're related to each other. A multi-tenant example scenario is also presented to illustrate the relationship between an application's application object and corresponding service principal objects.
+This article describes application registration, application objects, and service principals in Azure Active Directory (Azure AD): what they are, how they're used, and how they're related to each other. A multi-tenant example scenario is also presented to illustrate the relationship between an application's application object and corresponding service principal objects.
 
 ## Application registration