The answers in this section on autocephaly were provided by a seminary faculty member in a 1970 OCA publication.
Why is there such a distinction made between autocephalous and autonomous churches?
The only real reason for the distinction is that an autocephalous church is fully mature and fully capable of carrying on its own life, mission and hierarchal succession; whereas an autonomous church is not yet capable of doing so.
The autonomous church, however, even though it does not have the capabilities for full autocephaly, still has such unique conditions—geographic, linguistic, cultural, etc.—that it cannot merely be a “part” of some other church and be fully adequate to its churchly mission.
Thus, for example, the very particular conditions of the churches in Japan and Finland require that these churches have self-government and freedom from direction or control from outside. These churches, however, are not yet ready to be fully autocephalous because, for example, they do not have enough bishops to secure the normal succession of their hierarchies. Thus they are autonomous with a relation to another autocephalous church.
This, by the way, was the exact plan which the bishops in America were already asking for the Church in America at the turn of the century, more than sixty or seventy years ago. Even then it was understood that the conditions of the church in America were so unique that it could not possibly continue on as a diocese of the Russian Church which was thousands of miles away and whose conditions of life were completely different. Thus Archbishop Tikhon (later Patriarch Tikhon) and Father Leonid Turkevich (later Metropolitan Leonty) called for autonomy, and eventually full autocephaly, for America decades ago!