Question

The CURRENT channel in the mixer (as in FL Studio).
So the channel next to the Master, which can be activated like ListenBus. Measuring plug-ins can be dropped onto a channel and they will activate for the currently selected mixer channel(s).
Then you can put only one set of measurement plugs, after selecting the channel and listening, you can check the sound. For example, I have the Kick channel active, I check the kick on the docked spectrum analyzer from the Current channel. I select the Bass channel, the spectrum analyzer shows bass.

Answer 1

You load measurement plug-ins into the "CURRENT" channel, such as iZotope Insight, Span, Youlean, or PreSonus Spectrum Meter.
After selecting any channel (kick, bass, guitar / bus), the CURRENT channel and plugins attached to it are also activated.
Then you do not need to load the processor to load many measurement plug-ins on different channels, for example, to measure the spectrum or stereo image on the bass or synth channel, etc.
Who voted against, I have the impression that he did not understand the idea. If you don't want to use it, you don't have to enable this option. I've used this in FL Studio and it's very convenient.
You open the measurement plugin on the CURRENT channel and place it somewhere on the side of the monitor. And you see the audio measurement for each channel you are currently working on.

Answer 2

You mean Solo?