Boot-US considers partitions as bootable if the boot sector contains a valid
boot sector signature. Specifically the last two bytes in the boot sector
must be 55h AAh. When this signature is missing the corresponding partition
is considered non-bootable. By this simple check Boot-US decides whether a
primary or logical partition is bootable. Extended partitions are not bootable
in general, since they represent only the frame for the logical partitions.
Unfortunately, the presence of this signature is not a safe indicator of a
bootable partition. One could think that by checking the whole boot sector
one could decide whether a partition is bootable or not. It would "only" be
necessary to store a list of all boot sectors for all supported operating systems
and compare the actual boot sector to that list. However, this would mean a big
effort and even then it would not allow a safe identification of bootable partitions.
Even if the boot sector is fully correct the boot process can fail in a later
state, for example when a driver is missing or does not fit to the hardware.
Probably by now many user might have experienced this by themselves.
Therefore in practice the bootability of a partition can be verified only by
experiment, i.e. by trying to boot the partition. If it works the partition
is bootable, otherwise not. Thus partitions which are classified as bootable
by Boot-US could also turn out as non-bootable in reality.