Transmission fluid can safely be used in the power steering fluid pump without any problems and people have been doing this for decades and I've done it myself.
So when you run out of power steering fluid and all you have or can get at the moment and need to top off or refill your power steering fluid reservoir you can just use transmission fluid in the power steering pump and can even add transmission fluid to the power steering fluid.
What you can't do though is put power steering fluid in the transmission as power steering fluid in the transmission will damage the transmission.
But transmission fluid in power steering is fine.