Purpose/Usage of Boot Segments
Your question isn't very clear, but I'm going to take a stab at an answer anyway.

When a MCU comes out of hardware reset, the peripherals are generally in a quiescent state, but internal memory and registers (except for the PC and status register) are generally uninitialized.

However &mdash; and particularly when working with a high-level (compiled) language, most of the code is generated to work in an environment in which certain registers have well-defined values, and memory has certain data structures in place, such as a stack, a heap, and initialized global/static data.

It is the job of the "boot segment" to get from the first state of affairs to the second. It is written to operate in the minimal "environment" created by the hardware reset, and it creates the more structured environment that the application code expects.

If the MCU also has access to external memory, sometimes there's a memory controller that needs to be initialized &mdash; particularly if that memory is SDRAM &mdash; and then the in-memory data structures that reside there can be initialized, too. If executable code or initialized data resides in that external memory, this must be done before those application code and data segments get loaded.

The boot segment is normally included in the "runtime library" of whatever software development system you're working with, and you rarely need to deal with it. However, it might need to be configured with application-specific parameters such as memory controller register settings and the amount of memory to be allocated to stack, heap, etc.