Difference between solder mask and paste mask in gerber files

[Techetok]

What's the difference between solder mask and paste mask layers in gerber files? I don't quite get it and there is no obvious place to read about it either.
p.s. I'm not quite sure where this topic belongs, so I put it here

 

[FvM]

Have a look at arbitrary PCB boards in your reach. I'm sure you'll find on most boards solder mask openings that aren't printed with solder paste, e.g. all through plated pins.

Secondly, solder mask and paste pads undergo different size modifications according to technology requirements.

 

[Techetok]

So the paste mask should be smaller that the solder mask? In the image here

is the silverish thing solder mask and the spot-patterned thing on top of it - paste mask? Did I get it right?

 

[FvM]

Absolutely right. You also notice that the vias have an opening in solder mask but no paste printed on it.

The photo is showing a bad design detail, some pads next to a via without separating solder mask bar. Solder might drain into the via, resulting in unreliable solder joints.