What does SEG directive do in 8086?

Question

SEG A :

Assigns the content held in segment register corresponding to the segment in which A resides to the operand.

I guess that means

Answer 1

I guess that means that if A lies in Data Segment, SEG A is the same as DS.

Correct, if DS points to Data Segment.

does
MOV AX, LEA A
MOV DX, SEG A
MOV AX, [AX + DX]
copy the physical address of A to AX?

The last instruction is invalid, it does not exist in any of x86 CPUs. As such, this code does nothing at all. If anything, it just sits in an .asm file waiting to be corrected and assembled.