Regarding UV samples in MatterWaves:
When coordinates on the U and V emitter are positioned in the UV range of 0 to 1, (e.g. U = 10, V = 10), then in this case there will be 10×10 = 100 emitter positions.
If you choose to use material ID 2 then only some of the faces will be used, so the particle rate or shot will proportionally reduce to the whole size.
If you want to guarantee the Shot amount then turn on “Use Active Emitter only” since this distributes the whole rate on the selected part. Keep in mind you only have 100 individual emitter positions and therefore when the particle count is over 100 the positions will be re-used and particles will be born in the same place.