Okay, the user wants me to translate "sub-grade" into Persian. Let me think about the correct term here. "Sub-grade" can have different meanings depending on the context. In academic settings, it might refer to a lower grade or a subsidiary grade. In engineering or construction, it could relate to the foundation or base layer.
First, I'll check if "sub-grade" is already a recognized term in Persian. If not, I need to find the most appropriate equivalent. The direct translation of "sub" is "زیر" (zir) and "grade" can be "رتبه" (ratbe) or "سطح" (safeh). So combining them, "زیررتبه" (zirratbe) or "زیرسطح" (zirsafeh) might be options.
But I should consider common usage. In academic contexts, "زیررتبه" might be used for lower grades. However, in technical fields like construction, "زیرسطح" is more likely. Since the user didn't specify the context, I should go with the most general translation.
Alternatively, maybe there's a standard term in Persian for "sub-grade" in specific fields. I should verify if "زیررتبه" is commonly used. If not, perhaps "زیرگروه" (zirgroup) or another term. Wait, "sub-grade" in some contexts could be "زیرگروه" but that's more like subgroup.
Another angle: "sub-grade" as in a lower grade, so "رتبه پایینتر" (lower rank) but that's a phrase, not a single word. The user probably wants a single term.
I think the safest bet is "زیررتبه" as the direct translation, even if it's not the most common. Alternatively, maybe "زیرسطح" is better. Hmm. Let me check some sources.
Looking up "sub-grade" in Persian, I find that in academic contexts, it's sometimes translated as "زیررتبه" but in engineering, "زیرسطح" is used. Since the user hasn't specified the context, I'll go with "زیررتبه" as the primary translation, but I should note that context matters. However, the user asked for just the translation without explanations. So I'll provide "زیررتبه" as the response.
زیررتبه