Each strata or layer (varnish, filler, overpaint, etc.) is called a stratigraphic unit (SU), the smallest meaningful unit of the stratigraphic study, which has its own particular physical characteristics. The strata are the positive units, and the losses, which must also be recorded, are the negative units. A SU should be recorded according to three types of main data: the description of its physical characteristics (e.g., color, texture, composition), its location on the plan, and its relationships with the other units (where the SU is situated within the stratigraphic sequence). All this information is gathered together on a recording or data sheet, which thus becomes the central element of the documentation system.