Assuming segment is 32 bit

You have started a segment using the SEGMENT directive after having
enabled 80386 instructions, but you have not specified whether this is a
16- or 32-bit segment with either the USE16 or USE32 keyword. In this
case, Turbo Assembler presumes that you want a 32-bit segment.

Since that type of code segment won't execute properly under DOS (without
you taking special measures to ensure that the 80386 processor is
executing instructions in a 32-bit segment), the warning is issued as
USE32.

You can remove this warning by explicitly specifying USE16 as an argument
to the SEGMENT directive.