Note the above phrase carefully; it is extremely important to white lists. It simply means the level of trust the white list can confer to you, given your past behavior and records. In certain cases, they even share your trust level with individuals who ask. According to www.dnswl.org there are four basic trust levels.
High Trust Level: Never sends spam.
Medium Trust Level: Extremely rare spam occurrences, corrected promptly.
Low Trust Level: Occasional spam occurrences, actively corrected but less promptly.
No Rating Whatsoever: Legitimate mail server, may also send spam.
The above model is based on input by a team of volunteers. There is no fixed protocol. It is basically based on past performance and the web site which sends email has little or no input in changing its trust level.
Sending an email Diagram Source: www.wikipedia.org
Email Verification chart Diagram Source: www.wikipedia.org