The Control Room is an abstract way of managing your monitoring so that you don’t have to change target output connections on your hardware, nor on individual channels or sends to change between speakers, phone or cue mixes.
For your Focusrite, I would suggest that the connection arrangement would be:
a) Cubase Input channels:
_ a) Mic → In 01, named as ‘Vocal’.
_ b) Guitar → In 02, as ‘Guitar’.
_ Note that you can use the same physical inputs for multiple Input channels, which means that you could also have:
_ c) Stereo mic pair → In 1 + 2, named as ‘Acoustic’.
These can be set up differently in each Cubase Input channel, such as having filters, are then possible sources for Audio channels, but you have to make sure you correctly set the LINE/INST switch on the Focusrite for the Input channel currently being recorded.
b) Cubase Output channels are left not connected.
c) Control Room:
_ a) LINE OUTPUT 1 + 2 → Speakers, , as ‘Speakers’, using the BALANCED or UNBALANCED connections as required.
_ b) LINE OUTPUT 3 + 4 → Headphones, as ‘Phones’, but make sure the HEADPHONE SOURCE is set to ‘3+4’ on the Focusrite.
This enables you to use the Control Room to switch between the Phone and Speakers, but also allows having different plugins for each, such as having a room compensation plugin, such as IK Multimedia’s ARC, inserted into the speaker feed, or different EQ settings for each.
You do not have enough output channels on the Focusrite to provide any cue/foldback feeds. If you did, Control Room would allow you to put a reverb plugin (though advisably not too CPU-intensive) on the vocalist cue mix, while keeping what is recorded reverb-free.