1. 245(k) says if you are an employment based applicant or spouse or child of an employment based applicant we will forgive you up to 180 days of being out of status or on authorized employment since the date of your last entry. Any period accumulated before last entry we can disregard. We only count from the date of the last entry. Hence it is a very strong permission to help you file I-485.
2. 245(k) is automatically given. The government actually applies for it themselves. You don't have to do anything special for it.
3. When you are in between statuses: H-1B extension pending or old I-94 expired you are not in status enough to apply for this kind of status. But if your status expired less than 180 days ago because of section 245(k) you can file Adjustment of Status. More...
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