Class of Admission (Part 1, Question 11 on Form I-90) describes the immigrant category that was used to admit an immigrant to the U.S. as a permanent resident or conditional permanent resident.
You can find the three letter code on the front side of your green card right 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.
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 status (e.g. tourist) with the immigrant category under which you became a permanent resident.
Additional I-90 Resources