Some mcs51 opcodes address a byte like SBUF.
Other opcodes address a bit like SCON.1

The hex that is generated is 0x99 for either operand. However the preceding opcode has told the mcs51 which type of operand it needs for the particular opcode.

e.g. SETB SCON.1 // bit
e.g. PUSH SBUF // byte

You will notice that the only byte registers that will accept bit addressing have addresses like 0bxxxxx000.

If you try doing the wrong thing in ASM, you will get told off.
If you write in C, then the compiler looks after this sort of thing. Certain important SFRs have names for the bit-addressable bits. If there is no name in the header file, then it is probably not bit-addressable. So you have to use var |= (1<<x); construction.

The exception to this rule is that Ports P0, P1, P2, P3 are bit-addressable. ( and all the regular (non-special-function) registers )

David