Question about 5.4.2 in paper and reversible branch

[MinkiSong]

First, thank you for the excellent paper and code release. I have some questions as I would like to understand your paper and code more deeply.

Section 5.4.2 of the paper conducts an ablation study on PGI.

There are mentions of ICN, PFH, FPN, and PAN here. Could you provide references for ICN and PFH?
There are no descriptions of G_backbone and G_neck in Table 4. Could you explain how they differ?
In section 4.1, it states, "PGI mainly includes three components, namely (1) main branch, (2) auxiliary reversible branch, and (3) multi-level auxiliary information." Therefore, I understand GELAN + PGI as applying "auxiliary reversible branch" and "multi-level auxiliary information" to GELAN.

The performance in the repository's readme appears to be the same as the performance of GELAN + PGI in Table 5. Thus, I understand that all configs of yolov9 have "auxiliary reversible branch" and "multi-level auxiliary information" applied.

However, based on the conversation in issue #196, it seems that only yolov9-e has the "auxiliary reversible branch" applied.

Is the auxiliary reversible branch only applied to yolov9-e? If not, where does the "auxiliary reversible branch" apply in yolov9-S, M, and C?