Auto-detect is the default source mode. It is meant for mixed pages and short selections where you do not want to pick a language every time.
When to keep auto
Use auto for emails, tickets, and captions that change language without warning. The extension sends an auto source to the translator and reads the detected language when the service returns it.
When to set an explicit source
If short German lines are misread as another language, set the source yourself. Explicit source locks the request to that language code.
Second language
When detected text already matches your target, One Translate can switch toward your second language so you still get a useful result instead of the same text back.