Can someone help explain these resistors on an output transformer primary?

[Vyvyan]

Just wondering what effect the selectable resistances have on the output impedance. I'm use to seeing no termination on the secondary of the output transformer if any.

Obviously I here the change in output when switching. Is it possibly for better performance when driving different loads? ie 10k or 600r?

Btw the transformers are 1:1 600r nominal

 

[KIDVOX]

just looks like level selection to me but im no expert

 

[RoadrunnerOZ]

Output pads - simple voltage dividers.

 

[xpert2k22]

It is clearly mentioned in the schematic; the level selection between +4 and -6 for OUT A (L) and +4 and -20 for LINE OUT (L). +4 is International Studio Level standard and -6 & -20 are Domestic/Consumer Levels. The output impedance will remain the same 600 ohm on either selection.