Extension of stay is for foreigners holding non-immigrant visas such as B1 business visas, B2 tourist visas or F1 student visas to extend their stay in the US. How long a non-immigrant visa holder is allowed to stay depends on what they put on the visa application form and the final decision is determined by US Customs officers upon arrival in the US.
The “I-94 Form” can be found on the US Customs website and will have a date on it that states how long the visa holder is actually allowed to stay in the US. Thus, the date on the I-94 Form is the date that matters and not the expiration date of the US visa. Normally, an extension of stay applicants needs to have more than three months stay left on their I-94 Form in order to have time to apply for an extension of stay while in the US.
Foreigners waiting on extension of stay applications to process should leave the US before their visa expires. Any overstay no matter how short can result in being a US visa being in the future. Overstays of 6 months but less than one year will result in a ban from the US for a period of three years. Overstays of more than one year will result in a ban from the US for a period of ten years.