For multiple imputation, this is the number of completed datasets to create. The number of datasets created may not be the same as the number of data files. Say for example that you perform multiple imputation with three groups and request five completed datasets. Then if you select "Multiple output files" the program will create five completed data files for each group, for a total of 15 data files. If you select "Single output file", the program will create one output file for each group. Each file will contain five datasets, one after another.