I understand you're asking for a "deep paper" on the subject: — likely referring to SoundSource , a popular macOS audio control app by Rogue Amoeba.
: Users with older licenses (SoundSource 4 or 5) can typically upgrade for a discounted rate, such as $25 USD. soundsource license key work
A single license key is generally valid for a single user. If you own multiple Macs (e.g., a desktop and a laptop), you can use the same key on both, provided you are the primary user. I understand you're asking for a "deep paper"