If you open the Studio One Reference Manual and do a search for "metering," you will find some information on using the different metering systems inside of Studio One. There is a link on this page of the reference manual that sends you to the following pdf document that explains the new EBU R128 standard.
https://tech.ebu.ch/docs/r/r128.pdf
The red or orange box shows you the loudness range for your audio in real time.
To answer your second question, if you right-click the volume meter in the main output in the Console, you can switch the meter over to K metering while mixing.
Here is the section on metering from the reference manual:
Metering
High-quality metering is critical during the mastering process. The Project page offers three types of meters, each visible at all times, to help you make creative and technical decisions while processing your material.
Spectrum Meter
The Spectrum Meter is a flexible audio-spectrum meter that offers octave, 1/3-octave, 12th-octave, FFT, Waterfall (WF), Sonogram (Sono), and Segments display modes. The Spectrum Meter displays standard peak levels and can be adjusted to display Peak Hold levels for Short, Medium, and Long time intervals, as well as average (RMS) levels within Fast, Medium, and Slow time intervals. As you move the cursor around the frequency display, the note value of the current frequency is displayed.
The visible range of the meter can be changed in any mode, to help focus in on the range you're interested in. Do this by setting the Range controls, or simply by clicking and dragging vertically within the meter.
When using the FFT display, a -3 dB/octave line is displayed in addition to the frequency and level crosshair. This line represents compensation for the shrinking frequency-width of the FFT bands towards the higher end of the spectrum, which leads to a lower energy content. A well-balanced mix should somewhat approximate the slope of this line.
To disable the Spectrum Meter, click the "power" button under the lower left corner. Click again to re-enable the meter. To temporarily "freeze" the current state of the Spectrum Meter, click the snowflake button below the meter.
12th-Octave Meter
The bands in the 12th-octave meter correspond to the 12 musical tones in an octave, each in its appropriate place on a piano-like keyboard. This allows for easy reading of the pitch or note value of a given signal.
Level/Loudness Meter
The Level/Loudness Meter is located directly beneath the Spectrum Display and is capable of displaying high-resolution peak/RMS levels, three K-System scales (as described in K-System Metering) as well as the more recent EBU R128 standard. To choose a standard to view, click the selector below the Level Meter or [Right]/[Ctrl]-click within the meter and make your selection from the drop-down menu.
Nowhere is it more important to accurately meter levels than at the mastering stage of production. It is critical to be sure that the levels across all Tracks are as consistent as desired and that the signals are never clipped. When any amount of clipping occurs in your Project, a red clip indicator illuminates at the bottom of the Level Meter display, which can only be cleared by clicking on the indicator.
When Peak/RMS mode is selected, you can [Right]/[Ctrl]-click on the meter display to show additional metering options, such as RMS Length, VU Hold, and Hold Length.
To disable the Level Meter, click the "power" button under the lower left corner. Click again to re-enable the meter.
Loudness Display
Real-time numerical loudness information for the final output is displayed here, in LUFS (Loudness Units Full Scale, for absolute loudness measurements), or LU (Loudness Units, for relative loudness measurements). In either mode, you can see the Integrated loudness (INT), Loudness Range (LRA), and True-peak (inter-sample peak meter) reading. To reset the measurements, click [Reset].
Phase Meter
The Phase Meter, located to the right of the Level Meter, is helpful when checking stereo playback issues and mono compatibility. There are two components to this meter: a Goniometer at the center of the plug-in window and a Correlation Meter at the very bottom.
The Goniometer displays left- versus right-channel amplitude on an X/Y oscilloscope. A vertical line in the Goniometer represents a mono signal. The horizontal Correlation Meter compares the amount of in-phase and out-of-phase audio signal in the left and right channels. The parameters of the Correlation Meter range from +1 (mono signal) to -1 (reversed-phase mono signal), with 0 indicating the presence of totally independent signals (true stereo).
