You can find more information in the Command Line Tools User Guide, under the PAR Options section (page 122).
This option instructs the program to use results from a previous implementation to guide the current implementation, based on a placed and routed NCD file.
SmartGuide technology automatically enables timing-driven packing and placement in MAP (map -timing), which improves design performance and timing for highly utilized designs.
You can obtain better results if you use the map -timing option to create a placed and routed NCD guide file before enabling SmartGuide technology.
SmartGuide technology can be enabled from the command line or from the Hierarchy pane of the Design panel in Project Navigator.
-smartguide design_name .ncd
With SmartGuide technology, all guiding is done in MAP at the BEL level.
Guiding includes packing, placement, and routing.
SmartGuide technology optimally changes the packing and placement of a design and then routes new nets during PAR.
The first goal of SmartGuide technology is to maintain design implementation on the unchanged part and meet timing requirements on the changed part.
The second goal is to reduce runtime.
Note that the unchanged part of the implementation will not be changed and therefore will keep the same timing score.
Paths that fail timing but do not change should be 100% guided.
Paths that fail timing and are changed will be re-implemented.
The results from the MAP run are stored in the output map report file (.mrp).
Guide statistics, including the number of guided nets and all new, guided, and re-implemented components are listed in the map report, which is an estimated report.
The final statistics are listed in the PAR report file (.par).
A separate guide report file (.grf) is generated by PAR.
If you use -smartguide in the PAR command line, a detailed guide report file is created.
If you do not use -smartguide, a summary guide report file is created.
The guide report file lists components and nets that are re-implemented or new.