What are the differences between f1, f2, and f3 in the "forcing" component (denoted as "f") of the Variant Label?

[Kitty]

When I download the data, I usually choose the variant label 'r1i1p1f1' because there are more models under this label. I know that 'f' stands for the forcing dataset, but what is the difference between f1, f2, and f3? I can't find relevant documentation. Could anyone clarify this for me?
Thank you for any useful information.

 

[strandwg]

Specifically for the NCAR CMIP6 submission:

"f1" denotes the standard CMIP6 forcing for the given simulation.

"f2" was used for only two runs, CESM2-WACCM_G6sulfur_r1i1p1f2 and CESM2-WACCM_G6sulfur_r2i1p1f2:

G6solar_r1i1p1f1 branched from ssp370_r1i1p1f1 at 2015 and then solar dimming was applied after 2020
G6solar_r1i2p1f1 branched from ssp370_r2i1p1f1 at 2015 and then solar dimming was applied after 2020

G6sulfur_r1i1p1f2 branched from G6solar_r1i1p1f1 at 2020 and then feedback-control was applied after 2020
G6sulfur_r2i1p1f2 branched from G6solar_r1i2p1f1 at 2020 and then feedback-control was applied after 2020

The "f2" denotes the use of feedback-control, not solar dimming as was done for the G6solar runs. Apologies for the insufficient inline documentation.

"f3" wasn't used.

Other modeling groups may have other conventions for the value given for "f".