" Single, double, multiple" refer to the number of entries. For example, if you hold a single-entry visa, you can enter China only for once. "6-12 months" refers to the validity duration of the visa. For instance, your visa is valid for 6 months(from January 1 to June 30), you are suppposed to enter China before 24:00 of June 30. Otherwise, you can not enter China with that visa.