There are a few parameters in this behavior and they are all very simple to understand.
Width
This is the width of the render texture to which the camera is rendering. If it changes in run-time, and the Track Resolution parameter is enabled, the camera will regenerate the render texture and swap automatically. This option cannot be manipulated in run-time if the Track Resolution parameter is enabled.
Default: Whatever resolution the screen has.
Height
This is the height of the render texture to which the camera is rendering. If it changes in run-time, and the Track Resolution parameter is enabled, the camera will regenerate the render texture and swap automatically. This option cannot be manipulated in run-time if the Track Resolution parameter is enabled.
Default: Whatever resolution the screen has.
Resolution Multiplier
This is a multiplier applied to the screen size before generating a new render texture. Values up to 4 are possible, but you will most likely never be interested in anything above 1 (super sampling). This value is always tracked in run-time and a change will update the size of the render texture.
Example: If the screen resolution is 1024x512 (2:1) and the multiplier is set to 0.5, the render texture size will be 512x256.
Default: 0.25
Track Resolution
This parameter controls whether Width and Height are automatically tracked in run-time (see above).
Default: True