In partition table, we can give
/boot as mount point. But we can, too, toggle
bootable flag. What's difference between those two?
As for boot flag, see here. It is usually not needed for Ubuntu installation.
Boot mount point is the partition you mount your /boot to, if you use a separate /boot partition. If you do not have a separate /boot partition, /boot is just a directory under /. See here for more about partitions and mount points used in Ubuntu installation.
/boot mount point specifies the location to install the bootloader (GRUB, Syslinux), to be used by legacy BIOS-based systems in order to boot when the actual boot device contains a GUID Partition Table (GPT).
A boot flag, on the other hand, is a 1-byte value in a non-extended partition record, within a master boot record. Its primary function is to indicate to a MS-DOS/MS Windows-type boot loader which partition to boot.
If you have an MBR (Master Boot Record) type partition table, all you need to do is to set a boot flag on the root filesystem
/. However, if you have a GPT partition table, you need to create a separate BIOS boot partition and install GRUB to it.