Each paste mask layer is used to make a solder paste stencil for SMD assembly. The stencil is a thin metal sheet which - in a process similar to screen printing - is laid over the finished but unpopulated PCB and then a solder paste is be smeared over it, leaving a small amount of solder paste on each pad. The stencil is then removed and components are placed, retained by the solder paste, and the assembly is then put into an infrared reflow oven to melt the solder paste. More here: [https://www\.allpcb\.com/soldermask/solder\_mask\_paste\_mask\.html](https://www.allpcb.com/soldermask/solder_mask_paste_mask.html)