Let’s say you are reading a text file by using Data Flow task from a folder. If Data Flow Task completes successfully then you want to Run File System Task to move the file from Source Folder to Archive Folder. If Data Flow Task failed then you want to send email to your users by using Send Mail Task. The Precedence Constraints are used to control the execution flow.
Data Flow is the part of SQL Server Integration Services Package, where data is extracted by using Data Flow Sources ( OLE DB Source, Raw File Source, Flat File Source , Excel Source etc.). After extacting data Data Flow Transformations such as Data Conversion, Derived Column, Lookup, Multicast,Merge etc are used to implement different business logics and finally written to Data Flow Destinations (OLE DB Destination, Flat File Destination,Excel Destination,DataReader Destination ADO NET Destination etc.)