Complex sentence: Most modern scuba sets have a secondary second-stage demand valve on a separate hose, a configuration variously called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second".
Split into simpler sentences: Most modern scuba sets have a secondary second-stage demand valve on a separate hose, a configuration called an "octopus" demand valve. This additional second-stage regulator and hose is also known as "alternate air source", "safe secondary" or "safe-second".