ETL is a testing laboratory. They are similar to "UL" or "Underwriters Laboratories."
UL created a standard for safety for portable hot tubs, and ETL and UL
both use that standard to test for things such as:
- Max Temperature
- Electrical safety
- Bonding of equipment
- suction safety
- control system safety - and more
If you are getting a permit for your wiring, you will want to be sure you have selected a spa which is ETL or UL listed. Building inspectors know both - and to make it easier, ETL listed spas have a sticker: