The main difference between these two immigration forms is where you’re located at the time of your application. An experienced immigration lawyer can determine whether you should file the I-601 Form or the I-601A Form.
Form I-601
Form I-601 is the application for a Waiver of Grounds of Inadmissibility for those applying from a foreign country. You must file this form to apply for a waiver of inadmissibility, attend another consular interview in your home country, and wait for the USCIS to render an extreme hardship determination.
Waivers of Inadmissibility apply for people seeking entry into the U.S. to help loved ones suffering from health conditions or financial difficulty. For example, you may need to provide medical care for a sick family member, such as an elderly parent, who would face extreme hardship without your assistance.
Form I-601A
Form I-601A is the application for the Provisional Unlawful Presence Waiver. This type of waiver is intended for those who are currently in the U.S., have stayed without proper authorization, and are applying to stay in the country temporarily while waiting for approval to get a visa or legal status.
Essentially, you file this to remove the three- or ten-year bar on your entry due to living in the country without proper authorization.