While the Nintendo scene is the most common context, the term otp.bin exists in other, more professional engineering fields. It's worth a brief detour to understand the broader concept.
If you are developer or a power-user attempting to boot individual firmware modules, systemic data layers ( mlc.bin or slc.bin ), or run a comprehensive software-level firmware emulator, these two files act as the cryptographic ring of keys needed to parse the data safely. How to Safely Extract the Files
stands for One-Time Programmable . This memory type is physically fused (often using polysilicon or metal fuses) during the manufacturing process. Once a bit is flipped from 1 to 0 (or vice versa, depending on technology), it cannot be changed again.