TurboTax is following the wording used in the South Carolina Individual Income Tax Instructions. The amount entered could either be income or a gain, depending on the source.
According to these instructions:
Line h: Out-of-state income/gain
Enter:
● income from out-of-state rental property
● income from a business located outside South Carolina, or
● gain from real property located in another state.
Enter the amount as reported on your federal return. Check the appropriate box to indicate the type of income or gain.