How to understand `amd64DwarfIPRegNum` = 16?

[chainhelen]

 Hello all, I have some questions. 

Code here: 

amd64DwarfIPRegNum uint64 = 16

amd64DwarfSPRegNum uint64 = 7

amd64DwarfBPRegNum uint64 = 6

My understanding is that IPReg is pc register (in my mind) which represent instruction sequencer.     

But when I search https://www.uclibc.org/docs/psABI-x86_64.pdf, It describe `Return Address RA` which number is equal to 16.

 

My questions: 

1. How to understand `Return Address RA` ?

2. Why arm64DwarfIPRegNum uint64 = 32 ?

arm64 dwarf register here 

I will be appreciated if anyone can give me a answer.