It must be generally accessible. Usually type sections are set up in areas of well-exposed outcrop, but if, as in the case of subsurface units, they are defined in drill holes, the drill core must be held in a place where any geologist can come and see it. It must be representative of the unit. It should have the typical lithologies and boundary relationships of the unit. Ideally, all the lithologies of the unit should be present, although they are commonly not all present in a single section, and supplementary reference sections elsewhere may be needed.
Good exposure is required.
Usually the best exposure of the unit without structural complications is selected, that also meets the other necessary criteria. Sections may be offset so as to include more outcrop. If different intervals of a unit are only adequately exposed in different places, it may be necessary to designate one section as the type for part of the unit and another section as the type for another part. In this case, one of the component sections has to be designated the holostratotype or prime reference for the unit. If an unconformity is later found between the component sections, or the unit is otherwise split up, the holostratotype remains in the original unit by definition.