From Doomsday Wiki
The Include directive causes the parser to read the specified DED file. The contents of the included file are read immediately after the directive has been parsed.
#Include if the command line option '-texcomp' is not found on the command line. IncludeIf Not -texcomp "skin1.ded";
#Include if the identity key for the current game matches 'doom1-ultimate'. IncludeIf doom1-ultimate "udoomlights.ded";