L2 according to ISO 15957 dust goes by a variety of names. In ISO 12103-1 it is called A2 Fine Test Dust or sometimes "ISO Fine"– different names for the same dust! This dust is also specified in other standards including ISO 29461-1:2021 and ISO 16890:2016.
AC Fine and SAE fine are two designations for another type of test dust widely used in filter testing. AC Fine was introduced in the United States in the 1940s as a test dust for air cleaners. The Society of Automotive Engineers later adopted it as a test dust for air cleaners in their SAE J726 specification. However, the processes used to manufacture and characterize this test dust have undergone changes over the years.
Strictly speaking, AC Fine test dust and ISO Fine test dust do differ somewhat in their particle size distribution. This is something you should keep in mind when hearing the terms AC Fine or SAE Fine used colloquially as synonyms for ISO Fine, A2 or L2.