MOFA: A Guide to Japanese Visas - Documents to Be Submitted with Visa Applications
Instructor
Documents certifying the academic career or a copy of an educational license of the person concerned.
Documents certifying the professional career of the person concerned.
Basically what this amounts to is proof of your education and/or career as a teacher. Note that TESL certificates do not alone qualify you to teach in Japan no matter how they are promoted. A degree would be a different story because it is not a certificate.
Specialist in Humanities/International Services
Materials describing the business undertaken by the recipient organization.
A diploma or a certificate of graduation with a major in a subject relating to the activity of the person concerned, and documents certifying his or her professional career.
Proof that the company hiring is in the business of hiring teachers/ALT's. Now the second one about "major in a subject relating to the activity of the person concerned" is not enforced word for word. Why else would most ALT's I know be computer science majors? Basically all you need for this one is a degree which is why most teachers fall into this category. They're foreign, they went to school, why don't we make them a teacher?