A keepout layer shall host (normally a single) polygon. When there are multiple polygons on the layer, they shall not overlap or touch.
Different keepout types are represented by different keepout layers. In footprints the most commonly used would be ko@top-copper to avoid top side copper directly under some sensitive parts of a component. However, it is important to note that high end footprints shall assume solder mask present which means bottom side copper features on the component alone does not warrant a ko@top-copper since the solder mask would insulate. Better reasons are radio frequency (e.g. antenna) or magnetic (e.g. coil in a DC/DC brick).
Do not use ko@ to indicate mechanical keepout, use the courtyard layer for that.
Having a keepout layer in a footprint is rare. Do not improvise keepout layers: draw them only if the datasheet requested it.