Amusingly, there's a backward compatibility issue where the first name on a Let's Encrypt certificate can only be, at most, 63 characters long (because it gets

Amusingly, there's a backward compatibility issue where the first name on a Let's Encrypt certificate can only be, at most, 63 characters long (because it gets represented in the CN field, which is limited this way). All of the other names get represented only in SAN extensions, which don't have the same limit. So, if you need a Let's Encrypt certificate for a name over 63 characters in length, you have to also put another name on that certificate that is shorter, so Let's Encrypt has something to put in the CN field.