This update delivers improved general stability and compatibility.
Issues addressed with 9.1.2 include:
Improved compatibility with 6 and 12-core Mac Pro systems
Support for iOS control surface apps that utilize the OSC protocol
REX files now supported in 64-bit mode
Improved compatibility with select Audio Unit plug-ins
The update is recommended for all users of Logic Pro 9.
For detailed information on this update, please visit this website: Logic Pro Release Notes