This signature indicates the presence of at least a dummy boot loader which is safe to be executed, even if it may not be able to actually load an operating system.
This implies that FAT12/FAT16 media to be used also by very old versions of DOS must maintain the signature even if they do not contain an operating system or are meant to be bootable on other platforms only; consequently they must contain at least an x86 compatible (dummy) loader as well (for comparison, see examples of FAT on the Atari ST and with MSX-DOS).
Nevertheless, some media for other platforms erroneously contain the signature even without a x86 compatible dummy loader, making the check not 100% reliable in practice.
Some BIOSes (like the IBM PC/AT) perform the check only for fixed disk / removable drives, while for floppies and superfloppies it is enough to start with a byte greater or equal to 06h and the first nine words not to contain the same value, before the boot sector is accepted as valid, thereby avoiding the explicit test for 55h, AAh on floppies.
[1] From decimal offset 72 to 79, an NTFS PBR contains the partition UUID volume ID serial number.
The boot code in the VBR can assume that the BIOS has set up its data structures and interrupts and initialized the hardware.
The VBR is loaded at memory location 0000h:7C00h[1] and with the following CPU registers set up when the prior bootstrap loader (that is, typically the BIOS or MBR, but possibly another boot loader) passes execution to it by jumping to 0000h:7C00h in the CPU's real mode.
While this resolves to the same location in real mode memory, it is non-standard and should be avoided, since VBR code assuming certain register values or not written to be relocatible may not work otherwise.
Systems with Plug-and-Play BIOS or BBS support will provide a pointer to PnP data in addition to DL:[2][4] This information allows the boot loader (in the MBR or VBR) to actively interact with the BIOS or a resident PnP / BBS overlay in memory in order to configure the boot order etc., however, this information is ignored by most standard MBRs and VBRs.