Avidyne IFD540 Pilot's Manual page 62

A green chart extent box is separately drawn on the map and
represents the geographic boundaries of an instrument approach
plate associated with a FMS destination. When the ownship
symbol crosses the boundary of the chart extent box, this is a
good time to switch over to the Chart tab on the MAP page.
Chart Extent Box
The Map-FPL split tab on the FMS page presents the moving
map on the left and the FMS flight plan in a thin strip on the right.
This can be most useful to instantly correlate your flight plan with
the graphical depiction on the map. This aids in error reduction
and helps visually see options for diverts, weather avoidance, etc.
2-24 FMS Pages