I think I know what it is, but it may not be completely accurate.
My guess is that there is a library that must be included in the program specific to the device I'm using. Since I'm using the Atmel Studio IDE, the device is pre-selected and therefore I don't need to specifically include the device library? It's there, behind the scenes and the compiler takes care of the details?
For those unfamiliar with the phrase, I read it in the datasheet for the attiny, with regard to code examples:
3.2 Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the device. These
code examples assume that the part specific header file is included before compilation. Be aware that not all C
compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent.
Please confirm with the C compiler documentation for more details.