EMBR stands for Extended Master Boot Record. The EMBR specification dynamically extends the capabilities of the system.
The EMBR contains a block of code, called the EMBR Loader, and three tables — one for partition information, one for boot file information, and one for information on drivers that may need to be loaded on your system. Each of these tables can contain up to 255 entries.
The table in the EMBR that contains partition information is called the Master Partition Table (MPT). This table is used in place of the partition table to contain all partition information.
When an operating system is loading, at some point it looks for partitions in the partition table. At the very least it must find itself and typically assigns access to any of the other partitions or volumes that it recognizes.
BootIt BM handles the MPT and partition table by letting you create bootable configurations (or boot items) allowing you to choose which partitions from the MPT are to be placed into the partition table. If you are using the Limit Primaries setting, then all partitions from the MPT are automatically placed into the partition table. When the boot item is booted, the MBR partition table is updated to match the configuration of the boot item's MBR Details.
Note: You may find documentation from other sources which misuses the acronym EMBR in place of EBR (Extended Boot Record), which is an area inside of an extended partition.