Okay, the user wants me to translate "sub-grade" into Thai. Let me check the rules again. The first rule says to translate all content into Thai. The second rule says if it's already in Thai, leave it as is. The third rule is to provide only the translation without explanations.
So, "sub-grade" isn't in Thai. I need to translate it. The term "sub-grade" can mean a lower grade or a subsidiary grade. In Thai, "sub" might be translated as "ย่อย" and "grade" as "ระดับ" or "เกรด". So "sub-grade" could be "ระดับย่อย" or "เกรดย่อย". I should check which term is more commonly used in Thai contexts. Maybe "ระดับย่อย" is more appropriate here. Let me confirm that. Yes, "ระดับย่อย" is a common translation for sub-grade in academic or hierarchical contexts. I'll go with that.