I was reading a paper on Bayesian curve fitting (Dimatteo et. al. Bayesian curve-fitting with free-knot splines, 2001) and came across the symbol $\bumpeq$. It is used a few times throughout the paper but never explicitly defined. After a few google and stackexchange searches, it appears as though the symbol is neither widely used nor conventionally defined.
Below I give an example with context from the cited paper. I apologize in advance for not defining any of the other symbols, but doing so would amount to copying over large portions of text from the paper I have linked to and would be of little use to the question.
From p1059 (equation 8):
Incidentally, we can also see this in the likelihood ratio approximation for the normal model in equation (6) by
From context it seems that $\bumpeq$ represents an approximation. If this is case, then is it synonymous with more conventional symbols for an approximation like $\approx$ or $\sim$? or is it being used to represent a particular kind of approximation for which $\approx$ or $\sim$ would be insufficient or misleading?