The map files contain subsystems, and under various subsystems are the map-items themselves, which contain single values or arrays, either as integers or floating numbers. The items must first be created before anything can be put away, and various commands have been added to MAINDI (and other places) to facilitate this operation (look for commands with the word CREATE in them, such as MAINDI's "HIST CREATE"). Once created, the items are accessed by a unique key, which is tape*1000 + run, except for some of the spill-by-spill scaler items, which are accessed by seconds since 1-Jan-1970 (a standard reference date).
In general, when an item is requested from a map, the item with a key equal to or less than the requested key is returned. For example, if drift parameters are requested for key 17215026 (run 26, tape 17215), and they don't exist, the drift parameters for key 17169004 might be returned, if that's the closest "earlier" key for which the drift parameters are defined in the map.