The Flow Engine allows you to easily process your design and control the implementation of your design.
When you process your design, the Flow Engine translates the design file into the Xilinx internal database format (NGD). The Flow Engine then implements your design and generates bitstream data.
The Flow Engine allows you to control the process flow of your design in different ways. For example, if you are new to Xilinx software or just want to run a quick “sanity check” on your design, you can simply click the Run button at the bottom of the Flow Engine window to process your design. However, if you are an experienced user and want to fine tune your design, you can choose to modify your implementation, simulation, and configuration templates. The Setup menu contains these controls.
You control how far to process your design by using the Setup 
Stop After command. In the Stop After dialog box, select a process as your target break point. For example, if you want to map, place, and route your design but not create a timing simulation file or device programming file, select Stop After Place and Route. You will see a stop sign indicating where the process flow will stop.
You can create timing constraints, which control the implementation of your design. You describe these timing constraints in a user constraints file (design_name.ucf) or in the design source such as the schematic. In addition, you can include placement constraints in your user constraints file.
If you want to use a constraints file to control the implementation of your design, specify the user constraints file using the Setup Options command in the Flow Engine. In the Options dialog box, select a user constraints file. The implementation software tries to implement your design to meet the specified requirements.
