They're bits. SYN can be represented as 0x02, ACK can be represented as 0x10. 0x02 BITWISE-OR 0x10, ie SYN BITWISE-OR ACK or 'SYN-ACK' colloquially, is 0x12.
Those are just flags, the message contains much more than the flag. Therefore it is wrong to say that they just send the flags and that the flags are equivalent to SYN and ACK.