The solder paste (stencil) layer will only be used by whomever is assembling your board. If you are building it yourself by hand soldering, you won’t use this layer at all. This layer is what you would send to OSHstencils or Ohararp for getting laser cut kapton stencils. If you had an assembler building your board, they would (likely) use these files to get stanless steel stencils made.
So if you set the global value, it’ll just take whatever your pad size is and widen or narrow the pad based upon your global value. This will set how much solder will be layered on top of the pad in assembly.
As for the soldermask, this is also settable in the global dialog. This value is actually much more relevant to getting a PCB made. The soldermask layer (the purple stuff for OSHpark, green for many other types of PCBs) will determine where the copper is actually exposed. You can set this width because some manufacturers don’t want to put the soldermask right up to the edge of the copper pad.
Here’s a drawing, hope it’s helpful
