The .include line specifies that the named file is to be read and added to the input at the location of the .include line. Included files can be nested arbitrarily. The .include lines are processed before shell variable expansion is performed.
In WRspice, the keyword .spinclude is accepted as a synonym for .include. This is for compatibility with Xic, which will replace .include lines with the file contents, but will pass .spinclude lines to SPICE, after converting ``.spinclude'' to ``.include''.