If you are packing the publication to take it to a commercial printing service, you will see the options in the last window of the Pack and Go Wizard to print a composite proof or print separations. One or both of these will be checked by default depending on how the color printing is set for the publication.
If the publication is set up as RGB or single color, only the option to print a composite proof will be available. If the publication is set up as spot color, process colors, or process color plus spot color, the option to print separations will also be available. However, you can only print separations if you are set up to print to a PostScript printer.
You should print at least a composite proof that you can bring to printing service so that they have something to compare their output to. If you have a PostScript printer, you should also print separations. One or both of these proofs will help to establish to the printer what your expectations are for the completed job.
When you click Finish in this window of the Pack and Go Wizard, the wizard completes packing the publication. Also, if you are printing a composite proof or separations, the wizard prompts Publisher to print these using the original publication.