Basically, the new job must have the same essential job duties. The narrow interpretation would consider the position to be under the same category in the Dictionary of Occupational Titles or the O*NET published by the Department of Labor. This narrow interpretation is also the USCIS position. 

In practice, the USCIS has agreed that the AC-21 law does not limit it to an identical DOT or O*Net code, and has approved many cases throughout the local USCIS offices and the USCIS Service Centers, in which the new position does not exactly match the earlier job with respect to DOT Code or O*Net classification.