Typically if applying in the EA time frame then they would want the Loci. If applying in RD I haven’t seen this. But if you got a letter then follow what it says or /and reach out and ask. They are usually very nice and helpful. If sent to RD from EA then fill one out. They say on their website it’s all optional but I don’t consider too many things optional when showing interest. Keep in mind many many don’t send one in and are students at Michigan today. Good luck.