Line 1 - Legal name
Enter the legal name that matches the taxpayer identification number you provide. For a sole proprietor or disregarded entity, the IRS form says to enter the owner's name on line 1 and the business or disregarded entity name on line 2.
Common check: do not put only a trade name on line 1 if the TIN belongs to an individual or another legal name.
Common question: if you have a single-member LLC, should line 1 be your personal name or the LLC name? For a disregarded single-member LLC, line 1 is usually the owner's legal name; put the LLC or DBA name on line 2 when it is different.
Reddit SMLLC name/TIN discussion