Each member's rank is decided by the amount of Rakuten Super Points earned and the number of times they were earned.
Kindly note one of the conditions for becoming a Diamond member is to own a Rakuten Credit Card (available only for Customers in Japan).
For example, the first time you use it you become a Regular member in January, and if the number of points you have obtained by June is over 200 then in July you become a Silver member.
Other Ranks work in the same way: you can level up if you fulfill the stipulated number of points and number of times points earned within 6 months.
The terms for Keeping Rank for each member's rank are fixed as the same as the number of points and number of times points are earned as with Ranking upgrade.
For example, someone who is a Silver member in July has to earn points twice and the number of points has to be over 200 in the 6 months prior to July in order to not downgrade in Rank and to keep the same Rank in August.
Other Ranks work in the same way: you can Keep Rank by fulfilling the condition of minimum amount of points and number of times to earn points in the past 6 months as stipulated.