A peelable solder mask is a removable blue film that can withstand high temperatures printed on required PCB pads/holes at the end of PCB manufacturing. It serves to protect PCB pads/holes from oxidation and contamination. It is peeled off after SMT assembly and THT assembly. The size of the peelable solder mask is the same as the pads or PTH holes it covers.
Some electronic components are thermally sensitive and can't withstand high temperatures of reflow soldering and wave soldering, so they need to be manually soldered after the SMT assembly and THT assembly. However, without a protection film, the bare copper of the pads and holes will be oxidized under high temperatures during reflow soldering and wave soldering. And we should keep in mind that pads and PTH holes can't be contaminated by solder paste or tin. So we need to print peelable solder masks to cover these pads and PTH holes.
After SMT assembly and THT assembly, the PCBA is basically assembled except for mounting the thermally sensitive components. Then these components will be manually assembled on the PCB boards.