Sure: BCH is an abbreviation for “binary chart”. BCH files are constructed from the GSHHS chart data, which contains shoreline points in five increasingly detailed files, the most detailed of which is about 90Mb in size. Each BCH file contains the data from the corresponding GSHHS chart, recursively partitioned into a 2-dimensional tree to facilitate fast queries for the shoreline points contained in an arbitrary rectangular query area. The little window on the bottom of the display shows you which GSHHS file was used and how many points were retrieved for display in your current window.
The levels of detail, each in a separate file, are “crude”, “low”, “intermediate”, “high” and “full”. This is determined by the GSHHS consortium, not Bluewater Racing.
Bluewater Racing automatically chooses the most detailed BCH chart it can for display, subject to the constraint that not too many points are drawn in the current window. Right now, the constraining number of points is hardwired into the program. This is purely a time consideration, so that the program doesn’t spend too much time drawing points. You can override automatic selection by opening the preferences window and changing the chart setting from automatic to one of the five levels of detail files. This will force the program to only display shoreline data from that level of detail. Warning: if you force the program to use the full data set, and then zoom out so that the entire globe is displayed, you will wait a very very long time for the chart to get drawn, because there are some 10 million points.