Your systematic review is likely to contain references from multiple different sources, whether that is from files or different literature databases or from CuratorCR Repositories.
For example, upon import, you can tag the references with "Source = PubMed" if you were importing references from PubMed in the Find and Import References screen. If you are using the LitConnect tool, you can choose to automatically import references from Ovid or EBSCO, so you can tag the references with "Source = EBSCO" for example.
If you found references via citations of other references (citation mining), you could define the source as "Citation Mining" or label the references as such.
Both options are valid and easy to use for tracking this information. However, an additional benefit of using Labels to add this information to your references is that Labels can be used in reference partitioning with Filters.