A segment selector is the unique identifier of a segment and is used in the first part of logical address. See logical address syntax
It is a special pointer that identifies a segment in memory.
Its value can be found in a segment register.
When writing application code, programmers generally create segment selectors with assembler directives and symbols. The assembler and other tools then create the actual segment selector values associated with these directives and symbols. If writing system code, programmers may need to create segment selectors directly.
The value of a segment selector is hold in a segment register.
To access a particular segment in memory, the segment selector for that segment must be present in the appropriate segment register.
When using a segmented memory model, some instructions assume that pointers in certain registers are relative to specific segments.
For instance, some instructions assume that a pointer in the EBX register points to a memory location in the DS segment.