Today is the start of the 2018-19 International Signing Period, which means the Pirates and every other team in baseball can start signing players from the new class. The focus is typically on the players who just turned 16, or who will turn 16 in a few months, although there can be older signings who eventually turn out to be better prospects. As far as bonuses go, the biggest ones will typically go to the youngest player.
The Pirates haven't always gone after the highest rated players in the past, or the biggest bonus guys. There aren't really true ratings on the international side of things like there are in the draft.