Background: The Mississippian culture was a mound-building Native American civilization archeologists date from approximately 800 CE to 1600 CE, varying regionally. It was composed of a series of urban settlements and satellite villages (suburbs) linked together by a loose trading network, the largest city being Cahokia, believed to be a major religious center. The civilization flourished from the southern shores of the Great Lakes at Western New York and Western Pennsylvania in what is now the Eastern Midwest, extending south-southwest into the lower Mississippi Valley and wrapping easterly around the southern foot of the Appalachians barrier range into what is now the Southeastern United States. The Mississippian way of life began to develop in the Mississippi River Valley (for which it is named).
Context: The Mississippi stage is usually divided into three or more chronological periods. Each period is an arbitrary historical distinction varying regionally. At a particular site, each period may be considered to begin earlier or later, depending on the speed of adoption or development of given Mississippian traits. The "Mississippi period" should not be confused with the "Mississippian culture". The Mississippi period is the chronological stage, while Mississippian culture refers to the cultural similarities that characterize this society.  The Early Mississippi period (c. 1000-1200 CE) had just transitioned from the Late Woodland period way of life (500-1000). Different groups abandoned tribal lifeways for increasing complexity, sedentism, centralization, and agriculture. Production of surplus corn and attractions of the regional chiefdoms led to rapid population concentrations in major centers.  The Middle Mississippi period (c. 1200-1400) is the apex of the Mississippi era. The expansion of the great metropolis and ceremonial complex at Cahokia (in present-day Illinois), the formation of other complex chiefdoms, and the spread and development of SECC art and symbolism are characteristic changes of this period. The Mississippian traits listed above came to be widespread throughout the region.  The Late Mississippi period (c. 1400-1540) is characterized by increasing warfare, political turmoil, and population movement. The population of Cahokia dispersed early in this period (1350-1400), perhaps migrating to other rising political centers. More defensive structures are often seen at sites, and sometimes a decline in mound-building and large scale, public ceremonialism. Although some areas continued an essentially Middle Mississippian culture until the first significant contact with Europeans, the population of most areas had dispersed or were experiencing severe social stress by 1500. Along with the contemporaneous Ancestral Pueblo peoples, these cultural collapses coincide with the global climate change of the Little Ice Age. Scholars theorize drought and the reduction of maize agriculture, together with possible deforestation and overhunting by the concentrated populations, forced them to move away from major sites. This period ended with European contact in the 16th century.
Question: what exactly is chronology?

Answer:
The Early Mississippi period (c. 1000-1200 CE) had just transitioned