The relation among files, i.e. which files are needed to calibrate what, is represented as an xml tree. Xml is a rather common format nowadays and there are plenty of readers (and editors) available. For example, popular web browsers can be used to render xml either natively (e.g. Google Chrome or Firefox), or with a dedicated plugin (e.g. Safari). Popular scripting languages like python and idl have xml parsers.

The xml returned by CalSelector is a nested series of <association/> elements, each of which contains a list of mainFiles and the corresponding associatedFiles. This latter contains, in turn, another association and so on and so forth until the level of files that do not need calibrations is reached. 

Each fits file listed in the xml is assigned a type, which is suitable to be provided as input to the ESO pipelines (for more information on the pipelines, please refer to the relevant User Manuals).