Supplemental index is one of the two index categories (main index being the first one) where Google places certain webpages due to various flaws encountered in the indexing process.
Even though the webpage was relegated from the main index to a supplemental one, it continues to get indexed, with even fewer restrains in Google algorithm. This is the last stop for a page before is being dumped by Google.
Note that SI and Google Sandbox are two different issues; the last tends to appear after agressive backlink campaigns such as high volume on inbound links on short periods of time, disproportional with the website’s traffic, repetitive and “expensive” anchor text, anchor text changes, etc.
Also, consider that the index in which websites are included in are completely automated processes, thus there is no way of changing the index where they appear.
