Existe uma instância do Nautilus rodando nos bastidores que gerencia sua área de trabalho, então quando você executa instâncias subseqüentes do Nautilus, o --no-desktop está dizendo ao Nautilus para não tentar gerenciar os ícones da área de trabalho, etc.
O %U significa passar em uma lista de URLs:
%U A list of URLs. Each URL is passed as a separate argument to the
executable program. Local files may either be passed as file: URLs
or as file path.
O restante da lista pode ser encontrado aqui no "A seção da chave Exec" da documentação do freedesktop.org. Aqui está o resto.
trecho
Code Description
---- -----------
%f A single file name, even if multiple files are selected. The system
reading the desktop entry should recognize that the program in
question cannot handle multiple file arguments, and it should
should probably spawn and execute multiple copies of a program
for each selected file if the program is not able to handle
additional file arguments. If files are not on the local file
system (i.e. are on HTTP or FTP locations), the files will be
copied to the local file system and %f will be expanded to point
at the temporary file. Used for programs that do not understand
the URL syntax.
%F A list of files. Use for apps that can open several local files
at once. Each file is passed as a separate argument to the
executable program.
%u A single URL. Local files may either be passed as file: URLs or
as file path.
%U A list of URLs. Each URL is passed as a separate argument to the
executable program. Local files may either be passed as file: URLs
or as file path.
%d Deprecated.
%D Deprecated.
%n Deprecated.
%N Deprecated.
%i The Icon key of the desktop entry expanded as two arguments, first
--icon and then the value of the Icon key. Should not expand to any
arguments if the Icon key is empty or missing.
%c The translated name of the application as listed in the appropriate
Name key in the desktop entry.
%k The location of the desktop file as either a URI (if for example
gotten from the vfolder system) or a local filename or empty if no
location is known.
%v Deprecated.
%m Deprecated.