The first and most important thing to remember is: this letter and the I-601A Provisional Waiver ONLY — repeat ONLY — apply to a future immigrant if s/he is currently in the USA unlawfully, or has been unlawfully present in the USA in the past. Someone is illegally present if s/he enters the USA without inspection at a border post or airport, or if the person enters legally and then overstays the time granted on their landing permit.
If the future immigrant has never — ever — been in the USA, this letter and the I-601A waiver does not apply to him/her.
For some people who do indeed need the waiver — if they overstayed or entered without any papers — they should take a look at my blog from earlier this month.
If you have questions about whether you or a relative might need an I-601 or an I-601A waiver, it is very important to consult with a competent immigration attorney. –jcf