parallel/loop out
This means that the 2 RCA jacks are connected together. The 2 LEFT jacks are connected together (they are in PARALLEL) and the 2 RIGHT jacks are also. That means if you plug a cable from your DAC or phono preamp or whatever source into one jack, the signal appears on the other. The jacks are the same. The idea behind this: you can connect your CD player to your SRM-1 Mk 2 and then use an RCS cable to connect the other jack on the SRM-1 MK 2 to feed that signal to your receiver for your speaker setup. Those jacks are there to help you integrate the SRM-1 into your audio system.
Here's a schematic of how those jacks are wired. You can see they are just wired together. This allows the signal to "pass through" so to speak, so that you can arrange to feed the signal to your SRM-1 and also to another component, like your speaker rig. Or, in my case, I fed an SRM-T1 and then it was easy to compare the sound of the two amps, just had to move the headphones back and forth between the two.
So it doesn't matter which one you connect your signal to.