At the present moment, any return or onward tickets sold by China Railways will all take the form of additional Single Journey Tickets. (This appears to be the same as on, for example, National Rail in the UK, but with China Railways, these tickets are not linked as part of a Two-Part Return, for example.)
Please note that you cannot get return or onward tickets — as in those that involve departing a station that you are not physically in — at some counters. (Some passengers have reported that Rebooking counters cannot sell these tickets.) However, you are able to pick up pre-booked tickets via online booking or phonebooking at nearly all counters.
▶ More info: Getting single journey tickets