The code in the PBR can't read a file system's directory structure to locate this next piece of code to load. Such file-system-specific code would be too large to fit in the single 512 sector Instead a specific disk address for the next "chunk" is stored within the PPR.
As mentioned above, the code in the PBR is specific to the OS being loaded:
For Windows the PBR loads an additional 16 sectors from the beginning of the partition. These sectors constitute the first stage of the Windows boot loader which in turn loads one of two files depending on the version of Window:
For versions of Windows up to XP, the file NTLDR is loaded into memory. PBR then transfers to NTLDR.
For Vista and subsequent versions of Windows, the PBR loads bootmgr, the Boot Manager, into memory and transfers to that.
For Linux there are two popular boot loaders: LILO (Linux Loader) and GRUB (The Grand Unified Boot Loader). The latter is newer and has largely replaced LILO. For GRUB the PBR contains GRUB Stage 1. This code locates and runs the first sector of GRUB stage 1.5.
Next: Windows: NTLDR Linux: Grub Stage 1.5