Frequency tracking PLL type
There's a pretty good answer here, although it's at the bottom of the pile in points.

The short answer is that a type 1 PLL (i.e, phase detector feeding into the VCO) needs a phase error to drive the VCO off of its preferred frequency, while a type 2 PLL (i.e., a phase detector feeding a proportional-integral stage) will drive the phase error to zero, regardless of how much the VCO must be offset from it's preferred frequency.