Japan Reentry permit ( Sai nyukoku kyoka = 再入国許可 )
Assumed that you have status of residence in Japan.
If you have left Japan without any process, your status of residence becomes void.
Once your status of residence becomes void and if you want to reside in Japan again, you newly need to obtain COE and apply for acquisition of status of residence in Japan. So it will take several months to obtain it.
So such situation is not convenient for you who have some opportunities to depart from Japan.
So, before you depart from Japan, you can visit immigration office and obtain Reentry permit of Japan.
If you have received this reentry permit of Japan and return to Japan within designated duration, you can reside in Japan without application for COE and visa.
Special Japan reentry permit = deemed Japan reentry permit ( みなし再入国許可 )
In the case you have status of residence ( excluding short term ) in Japan, if you depart from Japan and return to Japan within one year, you are deemed that you have obtained Japan reentry permit.
So, you do not have to visit immigration office and obtain Reentry permit.
However at the time you depart from Japan, you need to clarify you have intention to apply for special Reentry permit of Japan.
You can not extend this one year while you are outside Japan.
On the other hand, in the case of ordinary Japan reentry permit, you can extend valid period while you are outside Japan.