For each supported system there's a header file that contains calls or defines specific for this system. So, when programming for the C64, include c64.h, for the C128, include c128.h and so on. To make the task for the Commodore systems easier, there is also a header file named cbm.h that will define stuff common for all CBM systems, and include the header file for the specific target system.
The header files contain
SID. Writing to the fields of this struct will write to the SID device instead. Using these variables will make your program more readable and more portable. Don't fear ineffective code when using these variables, the compiler will translate reads and writes to these structs into direct memory accesses.