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Gunter's Space Page says:

SJ 17 (Shijian 17) is a Chinese experimental satellite flown on the maiden CZ-5/YZ2 launch.

It is reportedly carrying out experiments with ion propulsion for station keeping.

The satellite will be directly inserted into geostationary orbit by the YZ-2 upper stage of the launch vehicle. (my emphasis)

CZ-5 has two stages according to Wikipedia, and second stage uses a pair of LH2/LOX restartable YF-75D engines. According to the Spaceflight 101 article China’s Long March 5 Heavy-Lift Rocket achieves full Success in Inaugural Mission:

The two engine nozzles began glowing bright orange again when the second stage re-started on a burn of just over four minutes to boost the stack into a highly elliptical sub-GTO-type orbit. Loud cheers emerged when the Yuanzheng-2 upper stage separated from the booster right at the half-hour mark into the flight, marking Mission Success for the Long March 5 rocket.

Assuming control of the flight, Yuanzheng-2 immediately fired its hypergolic main engine for a short supplementary burn to raise the high point of the orbit to Geostationary Altitude at around 36,000 Kilometers. Next was a coast phase of around five and a half hours to allow YZ-2 to climb to the apogee of the orbit so that its second burn can circularize the orbit and reduce its inclination to reach a Geostationary Orbit for spacecraft separation at 18:57 UTC.

The hypergolic-fueled Yuanzheng-2 seems to be considered an "upper stage", and yet, separation is considered the completion of the Long March 5's mission. Is the YZ-2 considered a 3rd stage?

How many stages does the Long March 5 actually have? Two, or three?

enter image description here

above: Long March-5 Rocket from here

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A Honda Civic is a two axle vehicle. It can tow a trailer. When it's towing a trailer, Civic-plus-trailer is functionally a three axle vehicle.

There's no essential difference between a spacecraft with propulsion and a rocket stage.

In this case, Long March 5 is a two-stage rocket because it's designed and built separately from the YZ stage.

Long March 5 + YZ is functionally a three stage rocket.

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  • $\begingroup$ A good point, though it would be interesting to distinguish whether the "upper stage" has some input into the control of events on the previous stages, such as separation, or is a pure passenger ejected by the previous stage. As an example, and please correct me if I am wrong, my understanding is that the Briz upper stage of the Proton follows the "control" model, delivered satellites follow the "passenger" model. $\endgroup$
    – Puffin
    Nov 5, 2016 at 15:09

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