pp.x

The purpose of pp.x is data analysis and plotting, which essentially extracts the data produced by pw.x and converts it to a file format readable by other programs.
By default, this will do a 3D plot in the XCrysDen format, but of course many other options are available and can be seen here.
The calculation type here refers to the plot_num variable.
No particular cuds structure is required here, but always remember to at least run pw.x or Quantum ESPRESSO will throw an error.