I suspected it wasn’t m/s, and did a quick check:
I set up a simple circle mesh emitter facing downwards and a domain with buoyancy density and heat set to zero in the smoke settings so the smoke didn’t go anywhere
With the circle emitter ‘initial velocity’ → ‘Initial Z’ set to -1 m/s:
The smoke took 100 frames to reach the bottom of the domain
And now the same setup but with ‘Normal’ set to 1 (and z set back to 0):
The smoke hasn’t been expelled half as forcefully.
Also, I think the tooltip on the ‘Surface Emission’ setting is incorrect, or at least seems misleading:
This to me sounds like it offsets the emission from the surface, when in reality, this controls the amount of emission from the surface
By contrast the tooltip for volume emission is worded correctly: