Similar to the {$EXTENSION } and {$LIBPREFIX } directives, the $LIBSUFFIX directive sets the suffix of the library. This is usually used for version numbers, which are simply added to the output filename, followed by the extension.
Example:
library tl;
{$LIBSUFFIX '-1.2.3'}
begin
end.
will result in a filename libtl-1.2.3.so on linux, or libtl-1.2.3.dll on windows.