The build string ( fbl_core1_kernel_npc ) indicates heavy work on the kernel and core system components. Under the hood, Microsoft was improving boot performance (precursor to "Fast Startup"), memory management for low-power devices, and early Hyper-V integration. Many of these changes would later define Windows 8’s performance reputation—for better or worse.
Compiled on January 24, 2011, Build 7899 represents the tail end of the development phase. It is a version of Windows that looks like the past but feels like the future. The Hybrid Aesthetic: Windows 7 Meets "Metro" windows 8 build 7899