User manual of HMC8015 seems to have a mix-up which left me puzzled for a while. Power is showed as negative instead of positive with wiring setup for voltage based measurement.
Consider that we are measuring AC or DC powered load. Power analyzer is connected in the middle between PSU and load as both ampmeter and voltmeter. Instantaneous power is P[W] = U[V] * I[A], it can be positive (V and A signs match) or negative (V and A signs don't match). Sign flip, if needed, is solved in menu setting.
There are two ways to combine ampmeter and voltmeter connections, both are provided in manual. "Voltage based measurement" does not include voltage drop over ampmeter, "current based measurement" includes ampmeter voltage drop.
For HMC8015, ampmeter and voltmeter have common ground (COM) - this means that "voltage mode" would actually yield negative power calculation while current mode would yield positive (see schematic with setup and signs).
However, user manual states the opposite - as if connected in voltage mode power would show positive P (while current mode would show negative P and require sign flipping). Pictures and/or descriptions are mixed up in user manual (see pages 13 & 14).
3 wire connection somehow is more complicated than 4 wire, even R&S slips up. No biggie, manual can be updated.