Since early versions of Sibelius, there have been Default Symbols and Lines. In the case of both Symbols and Lines, some of these are hard wired to specific functions in the program, and in the case of Lines in particular, these attributes are inherited by any “New” version you might create of that Default line.
For instance, the Default 8va line has an effect on playback. If you make a copy of this line by selecting it and clicking the “New” button, the New copy will inherit the same playback attributes. In the same way, if you change the visible attributes of the default 8va line, say, to different preceding text or change the line thickness, it will still maintain those playback characteristics.
But the ability to edit the defaults directly has some ramifications.