Although this is not a formal description regarding the Celestia implementation of constellation boundaries, this answer may probably help you further.
I used the formal IAU site to figure this out:http://www.iau.org/public/constellations/
The IAU page contains a description of their format:
For each constellation, there is a TXT file, describing the boundaries.
The text file contains a set of coordinates that defines the boundaries of the constellations in the sky.
The format is: HH MM SS.SSSS| DD.DDDDDDD|XXX
HH MM SS.SSSS defines the right ascension hour, minute and second with J2000 coordinates
DD.DDDDDDD defines the declination with J2000 coordinates
XXX is the abbreviation of the constellation name
| is the separator of the fields
22 57 51.6729| 35.1682358|AND
Celestia however, does not use the HH MM SS.SSSS notation for right ascension, but the minutes and seconds are defined in decimal values.
In Celestia the declination is the same format, using a + or - sign
In celestia the abreviation of the corresponding constellation is the same (I did not check all, but I think so)
Celestia uses a space separator instead of the | character
And finally in Celestia each row contains a "O" or "I" character.
As far as I can figure out, the "O" defines the start of a line and the "I" defines subsequent points on that line.
Each time a new "O" is given, you can see a direction change in the Constellation boundary.
Using the images on the IAU site, you can check it further out yourself.
Kind regards, Marco