Class of Admission (Part 2, Item 40.a. on Form I-130) describes the immigrant category that was used to admit an immigrant to the U.S. as a permanent resident or conditional permanent resident.
The Class of Admission code can be found on the front side of your green card under “Category.” It is typically one or two letters followed by a number, (e.g., IR2). For example, RE8 would be the class of admission for the green card holder shown in the image below. The location on the card has changed over time. It could be located in a different place on older cards. For examples of more types of green cards, use CitizenPath to prepare your Form I-130.
Note: You may have entered the country on a non-immigrant visa and adjusted status to permanent resident. In this case do not confuse your initial nonimmigrant status (e.g. tourist) with the immigrant category under which you became a permanent resident.