The schema file in App_Code/DAL/Members.xsd defines the typed data set that is used to insert members into the classifiedsdb.mdf and get members by user name. When the typed data set was created, the namespace MembersDataComponentTableAdapters was created with all the auto-generated code to access the typed data set.
In the App_Code/BLL/Members.cs code file, the CreateMember method uses one of the queries on the data set (the InsertMember) so a using statement with that namespace permits the use of types from that namespace.
For details on the DataSet Designer see:
http://www.theserverside.net/tt/articles/showarticle.tss?id=DataSetDesigner
http://weblogs.asp.net/scottgu/archive/2006/01/15/435498.aspx
http://msdn2.microsoft.com/en-us/library/314t4see(VS.80).aspx (general dataset designer links)
http://msdn2.microsoft.com/en-us/library/ms233697.aspx (has a blurb about the naming convention)
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