To view cross-sectional cuts of your model, you can enable up to six section planes. When a plane is ‘enabled’ it means that it affects (cuts through) the scene. The current plane is the one that is rendered visibly in the Scene View. Selecting a plane as current automatically enables that plane if it was not already enabled.
When a section plane is first enabled, it is created with the default alignment and position. Afterwards, enabling a section plane restores the saved alignment, position, and rotation if it is available for the current viewpoint.
By default, a section plane is created within the view, and as close to the center of the view as possible. Visually, a section plane is represented by a light blue wireframe. You can hide the visual plane representation by toggling the corresponding gizmo button.