Movements down to the next lower level are always easy, because special function keys (buttons) are offered at the bottom of the higher-level page (web interface).
A PI can choose one of the proposal drafts from the list left in memory previously by clicking on the proposal title. At the next level (the observation level), two buttons are offered named ``Add Co-I'' and ``Add Observation''. The ``Add Co-I'' button is used to enter personal information for each Co-Investigator on the proposal. The ``Add Observation'' key leads the user to the level where information about the target of a proposed observation is entered.
One level lower, function keys are offered again to ``Add Time Constraints'' and ``Add EPIC MOS'', ``Add EPIC pn'', ``Add RGS'', ``Add OM''. The first is used to enter time constraints, if any. The latter four allow the user to enter exposures for the different types of XMM science instruments. Note that there are two EPIC MOS cameras and also two RGS detectors, for each of which exposures must be defined. This is the level of input at which the schematic with the pre-planned sequence of exposures within an observation (§ 3.1) comes in handy.
In addition to the above function keys, two more are available at this level, namely ``Copy Observation'' and ``Delete Observation''. The names are self-explanatory.
At the end of each exposure page one also finds the functional buttons to add exposures or to copy/delete the current one. These operations do not lead to a lower level, but create or remove entries at the same (i.e., the exposure) level.