What do the (+) and (-) symbols after variables mean? This paper describing an Unscented Kalman Filter implementation uses notation that I am unfamiliar with nor can find on eg https://en.wikipedia.org/wiki/List_of_mathematical_symbols 
Xu et al (2008)
An example line is:
$$\chi_{i,k-1}(+) = f(\chi_{i,k-1}), x_k(-)=\sum_{i=0}^{2n}\omega_i\chi_{i,k-1}(+)$$
This notation continues throughout the paper.
Could someone please tell me what the (+) and (-) indicate? At first I thought it was redundant information describing the post and prior versions of $x$ however one of them is $\chi$ and the other is $x$. I also thought it could be the positive and negative solutions of a square root but that isn't obvious to me as being correct either (why (+) of $\chi$ and (-) of $x$ and where are the other solutions).

They seem to refer to the a priori and a posteriori estimate. Compare the equations with these, Wikipedia uses $x_{k∣k−1}$ where your paper uses $x_k(−)$ and $x_{k∣k}$ instead of $x_k(+)$.
