What is the "Bootable flag" option when installing a distro?

The boot flag is from ancient times, where you would indicate an MBR partition record as bootable, so you could indicate where the boot loader resided.

On modern OS'es this is widely unused, as the MBR consists of a minimal stage loader which bootstraps either into its own partition or jumps to another area on the disk where the boot loader code is kept. (An MBR can contain either executable code or the boot partition table among other things. See also this link to an article about the MBR).

As an example, GRUB is written into the MBR and boots whatever partition you choose.

See also this (quite small) Wikipedia page about the boot flag: en.wikipedia.org/wiki/Boot_flag

At first I thought it's a bug in grub2. Some BIOS seem to test if at least one partition is marked as bootable. After a long research I noticed that, because there was no way to boot from hdd or usb-stick at all. That means even if magic bytes at the end of mbr are correct and boot-code in mbr is valid, BIOS will ignore that device and skip it in boot-sequence until bootable flag for at least one partition was set. Tested on Fujitsu P772 and HP Probook 6570b.