Some questions about the Cortex-M3 bit-banding feature

(I am new to ARM microcontrollers. Pardon me if this is a naïve question. )

ARM-M3 processor provides a unique bit-banding feature which maps a bit in the first 1M address range of SRAM and peripheral regions to a 32-bit word in another alias address ranges, respectively.

My questions are:

• Why only map the first 1M? What's so special about this first 1M range?

• The mapped 32M alias ranges are still in the SRAM/Peripheral range, so they cannot be used for other purpose, right?

• If I want to set a bit in the bit-banding region, should I write 0xFFFFFFFF, 0x000000FF or just 0x1 to the mapped alias word? It seems all of them can work but I am not sure which one is safe/correct.