回複：急問:I-485 180天後換工作 -> 什麽是

Assuming you're asking about changing job with a LC-based I-140, AC21 applies. The following three factors are used to determine if the new job is a similar job:
1. Job duties (expected to have >75% overlapping with the approved LC job description)
2. Job code using DOT or SCO job codes
3. Salary difference

Just remind that a lot of people are confused AC21 with job change with a 485 based a NIW or E1A-based I-140. The latter has much more freedom than AC21 as it only requires an applicant to work in the same field claimed in I-140.