While the "+"s and "-"s are most often used to prioritize trips into (up to) 7 different pool priorities, they also perform this function. It can get complicated. From the PBS Guide,

"When you put trips into any of the three High pools, you are instructing PBS to use as many of those trips as possible to build your line. Conversely, when you put trips into any of the three Low pools, you are instructing PBS to use as few of those trips as is necessary to build your line."