The CSN identifies the Apple SIM. An Apple SIM can store a virtual SIM card, or eSIM. The eSIM has an ICCID number, like standard SIM cards, but as the iPad may download a different eSIM in each country, the ICCID may change, but the CSN stays the same.
If you're asked to find your CSN, this is what you do:
- Go to Settings > General > About
- Tap the ICCID field - the field changes to show the CSN