Tier memberships work based on spend or approved points earned over a 12 month period. Each tier has a spending range, e.g. $0 - $99, $100 - $499, etc.
To illustrate with an example. We have the following three tiers:
We have a shopper who registers and spends $50 on day 1. She will be in the Bronze tier, as her spending has not reached the threshold for the Silver tier.
On day 150 she spends an additional $75. As soon as she completes her purchase, she will be moved into the Silver tier, because her total spend in the last 12 months is now $125. If nothing else changes, she will remain in the Silver tier for 12 months, i.e. her Silver tier membership would expire on day 515.
Let's look at a few different scenarios now:
1) She doesn't purchase again. On day 515 her Silver tier membership will expire and she'll be moved back into the Bronze tier.
2) On day 250 she spends $400. She will now be moved into the Gold tier as her total spend in the last 12 months is now $525. She will remain in the Gold tier for 12 months.
3) Let's assume that the shopper then stays in the Gold tier for 12 months (days 250 - 615) and during that period spends $200 on day 500. On day 615 her Gold tier membership will expire because she will have only spent $200, not the $500 needed to be in the top tier. Our shopper will then be moved into the Silver tier because her spend in the last twelve months is $200, in the $100 - $499 range. She will remain in the Silver tier for a further 12 months, until day 980.