In both Project Mode and Non-Project Mode, incremental implementation mode is
entered when you load the reference design checkpoint using the read_checkpoint -incremental <dcp_file> command where
<dcp_file> specifies the path and file name of the reference
design checkpoint. Loading the reference design checkpoint with the
-incremental option enables the Incremental Compile design flow for
subsequent place and route operations. In Non-Project Mode, read_checkpoint -incremental should follow opt_design and precede place_design.