This button automatically sets the "Default Focus" and "Default Zoom" fields, which define the focus location and zoom level that the map "snaps" to when the record is selected. For example, if the record is represented by a vector annotation on the map, and the user clicks on the title of the record in the Waypoints panel, the map will move to the location and zoom level defined by these fields. Think of these fields as a related pair that work together to set the record's "framing" or "home base" in the exhibit.
To set a default focus and zoom, just move the map to the exact location and zoom level that you want to use, and click the "Use Current Viewport as Default" to insert the current values into the inputs.
If no values are provided for these fields, Neatline will fall back on an automatically-computed focus location by centering the map around the geometric extent of the record's vector annotations on the map. This can often work well without any modification, but there are a couple of cases when you might want to explicitly set a custom focus:
If the record is represented with a single point, Neatline will always zoom in to the highest possible zoom level, since points are effectively dimensionless, infinitely-small dots. This is often undesirable. To fix, just zoom the map back to a reasonable level and click "Use Current Viewport as Default."
Even when the record is represented by lines or polygons that result in a fairly reasonable automatic focus, there are times when you want to tweak the default viewport, perhaps to situate the record in the context of some other nearby feature on the map. For example, imagine that the record represents the location of a battle that was affected by the presence hill to the north of the battle site. You might want the default focus of the record to be zoomed back a bit and moved up to the north so that the hill appears in the default viewport of the record for the battle.