Determines when the statute of limitations begins running. It is fixed by the date that the disseisor entered the land (and technically, all of the elements for AP are satisfied; assume they are all satisfied until told otherwise).
Merely walking on land, or hunting, does not establish actual possession. His actions must (usually) change the state of the land. Examples include, clearing, mowing, planting, harvesting fruit of the land, cutting timber, mining, fencing, pulling stumps, running livestock and constructing buildings and other improvements.