The standard working space is Rec2020, which has a white point of D50.
white balance with the reference D65 is used to give a somewhat wrong, but ‘better than nothing’, approximate white balance, because some modules rely on the colour being at least approximately correct. color balance can then re-read the camera’s WB setting, and make sure not D65, but the actual illuminant white point is mapped to the working space’s D50 white point. At least that’s how I understand it.
You may find the colour temperature (CCT) readings in color calibration are not what you expect. I think the reason is what I described here: Confused about D50, D65, and CCT in white balance and color calibration modules - #10 by kofa.
Also, as you are a newcomer: do not worry, if the color calibration module says the CCT is invalid.
When the CCT is followed by “(invalid)”, this means that the CCT figure is meaningless and wrong, because we are too far from either a daylight or a black body light spectrum. In this case, you are advised to use the custom illuminant. The chromatic adaptation will still perform as expected (see the note below), so the “(invalid)” tag only means that the current illuminant color is not accurately tied to the displayed CCT. This tag is nothing to be concerned about – it is merely there to tell you to stay away from the daylight and planckian illuminants because they will not behave as you might expect.