Labels don't require any memory.
0;JMP // goto output_d
(OUTPUT_FIRST)
@0
The 0;JMP is stored at ROM address 9 (0000 1001) and the @0 at 10 (0000 1010). The symbol OUTPUT_FIRST is given value 10, so @OUTPUT_FIRST generates machine code 0000 0000 0000 1010.
--Mark