Sound design is the art of creating auditory elements for the various media. It involves the production and development of sound and soundscapes with the intent of eliciting a particular aural experience.
sound-design is naturally quite a broad tag on Sound Design Stack Exchange. Use it when the question is directly related to the process of designing sound and soundscapes through production, recording, and development, and the problems and learning curves therein. Where no other more specific tag is suitable.
Example: Microphone placement, even though it may be a part of the sound design process, the microphone tag would be a better broad tag choice, then microphone-placement for a more specific tag.
But if you want to know how to create a specific sound using an additive synthesis model, then use sound-design along with synthesis.
Another way to look at it, is; do you think your question should show up in a "sound design" tag search?