Proposal [1]
The board square color flag concept was firstly mentioned in PDN 1.0 specification in "GameType tag", more precisely defined by Invert-flag attribute.
Since PDN 3.0, its meaning changed to indicate if the bottom left corner is or not a playing square.

On the one hand it can represents the board beginning position patterns of the most draughts variations, because the pieces square usually is black. On another hand, some exceptions can exist, for example the "Traditional" Spanish draughts variation, where the piecies are on white squares, instead of "Modern" Spanish draughts / Portuguese draughts.
In addition, for example in Ossetian draughts, the pieces can be placed on grid intersection points.

The main idea is represents the bottom left "square" color, thus the board setup based on PDN file can offers a representation closer than real-world board patterns.

Example:
0 - black square color
1 - white square color
2 - uncolored square
3-  grid intersection point