You should use the pin labels that are printed on the device, that way everybody knows what you are talking about (irrespective of how you look at the device ) and that’s what the silk screen labeling is for.
This is the official image (https://docs.particle.io/assets/images/photon_vector2_600.png)
When you use these labels you can also answer the question by means of the docs as they also always talk these pin labels.
The comment block in the library files states
* Soft RX pin needs to be interrupt enabled, so on Photon D0 & A5
* won't work as RX
The sample states
#define PSS_RX D2 // RX must be interrupt enabled (on Photon/Electron D0/A5 are not)
And here is the background to that limitation
But if you are using RX/TX there is absolutely no need to use
ParticleSoftSerial since these pins are the hardware UART interface pins which can be used simply by means of