Head Phone vs. Low Level Audio

I am helping a friend wiring his very old Narco COM 120 to a SPA 400 intercom. The only documentation on the Narco is a pinout from AeroElectric.com

http://www.aeroelectric.com/Installation_Data/Narco/COM%20120/COM120-Pinout.pdf

There are a few pins that are very confusing to me:
3 head phone
5 speaker
7 aux audio in
8 low level audio out

Which COM 120 audio output (3, 5 or 8) should I use as com audio input on the SPA 400?

I have seen installations when using a speaker output to drive the SPA 400 com audio input a resister was used in between. But why is there an inter connection between 7 & 8 with a resister?

If I use 8 as input to SPA 400, do I need the inter connection between 7&8 with a resister?

Thanks in advance for your help.

I may have just found the answer to my own question:

https://www.sigtronics.com/_pdf/air_pdf/air_intercom_pdf/SPA-400_Com-120_Inst.pdf

However, I appreciate very much if anyone can explain what is the purpose of the interconnection between pins 7&8.