There are two types of re-entry permits: “Re-entry permit” and “Special re-entry permit”.
I will explain characteristics of each.

Re-entry permit

1.Why do you need a re-entry permit?
By obtaining a permit, you have made a kind of declaration of intent,
In other word, it is a statement of intent to continue to reside in Japan with the same status
of residence after returning to Japan.
This procedure simplifies the landing examination process.
To obtain this permit, you must apply for it at the immigration office before leaving Japan.

2.Who needs to get the permit?
If you plan to stay outside of Japan for 1 year or more and return to Japan again, you will
need to obtain a re-entry permit. (2 years or more for special permanent residents)
If you depart from Japan without this procedure, you will lose your period of stay and status
of residence.
This is the same even if you are a permanent resident or special permanent resident, so be
careful.

3.Re- entry permit is valid for 5 years. (6 years for special permanent residents)
The re-entry permit is valid for 5 years, and you can extend only once in case of
unforeseen circumstances.
The extension period is limited to 1 year.
To sum up, from the date that the re-entry permit becomes valid, you can stay outside of
Japan for up to 6 years. (7 years for special permanent residents)
You must do the extension procedure by yourself at Japanese embassy or consulate.
Please note that you cannot extend it beyond the expiration of your status of residence.
According to the provisions of the Immigration Control and Refugee Recognition Act, an
extension of the valid period can be granted when there are reasonable ground to do so.
In short, you should assume the limited situations such as the serious illness or injury, or
being involved in a war.
There are two types of re-entry permits: one that is valid for one time only, and one that
allows you to leave Japan as many times as you like.

Special re-entry permit

If you re-enter within 1 year, you will be granted a special re-entry permit.
Once you obtain a special re-entry permit, you do not need a regular re-entry permit,
When you leave Japan, check the box on the Re-Entry Departure Record (ED Card) and
tell them that you want a special re-entry permit.
Note that the special re-entry permit cannot be extended, unlike the re-entry permit.

Summary

Re-entry permits simplify the immigration process.
One more thing, obtaining a permit has another effect.
Another effect is that you are considered to have continued to reside in Japan during the period of your stay abroad.
That is, it means that it can also cover the requirements of continuous residence that will be
required when applying for permanent residence in the future.
In that sense, it is important to obtain the re-entry permit.