The announcement below from Japan sounds like one becomes an assistant professor at the end of the tenure process. I thought the tenure is always meant for becoming associate professor.
Terms of Appointment:
Evaluated in the final year of the term, and if satisfactory, eligible to become a tenured assistant professor as of April 1, 2018.
Japan's academic system is still closer to the continental system where there is only full Professor (教授） in a department and several assistant professors (助教授）and a mixture of adjunct and full-time readers/lecturers (講師）. The assistant professors can be tenured.
Some schools in Japan are moving to the American system where there can be several full professors in a department and there are now some colleges that are introducing the category of associate professor (准教授）).
Japanese labor laws tend to protect full-time faculty, so you can assume that you have lifetime employment if you pass the first employment review -- if and only if you haven't been hired as a part-time (非常勤) or otherwise term-contract employee (非正規雇用).
In this case, it sounds like they want to emphasize that you are NOT being hired as part-time or other non-tenure track (such as the new "jokyo" 助教 which is not the same as a tenure-track assistant professor 助教授).
Tenure is not always associated with the traditional assistant vs. associate/full distinction, especially not outside the United States. Even in the United States Associate does not always mean tenure nor does some other title necessarily imply untenured.
If you're applying for this specific position and it concerns you, you should probably just ask for clarification.
