The courses that I've taken that were very basic and introductory was named 100. The next one was perhaps 101. Introductory courses in more specialized courses

The courses that I've taken that were very basic and introductory was named 100. The next one was perhaps 101. Introductory courses in more specialized courses often had numbers signifying which research group they belonged to, with a low number at the end. Practices vary, even at my own university. I'm guessing that 101 specifically is an americanism.