S.C. Code Regs. § § 117-710.1
The phrase "transacting or conducting his business partly within and partly without this State" as used in Section 12-6-2210, is applicable to a single business operation, which is unitary or homogenous and is carried on both within and without the State. A taxpayer operating two or more unrelated businesses, each of which is entirely within and without the State, is not subject to the provisions of this section, but each business determines its South Carolina net income separately. A taxpayer operating a unitary or homogenous business within and without the State and an unrelated business either entirely within or without is subject to the apportionment formulas with respect to the unitary or homogenous business but not with respect to the unrelated business. The income from the unrelated business is allocated and apportioned separately as appropriate to the State where such business is conducted.
S.C. Code Regs. § 117-710.1