It's all SIC, but you can log it as "SIC acting as PIC" for the purposes of logging 250 hours of PIC time for an ATP.
FAR 61.159.a:
(4) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties of pilot in command while under the supervision of a pilot in command, or any combination thereof, which includes at least—
(i) 100 hours of cross-country flight time; and
(ii) 25 hours of night flight time.
So you can't log it as PIC, but you CAN count it towards the ATP PIC Requirement.