The IO cards have various outputs. 

• Switched Ground
• 3V3 output
• VBoost Output (can be configured to 5V or 12V)

For specific IO mappings on each Card, please see this article.

Generally, the 3V3 output is used to power I²C sensors when taking a reading. And the VBoost for higher-powered sensors like SDI-12 or 0-5V Analogue sensors. 

The switched ground is typically used to control a pump/relay/motor etc. 

It is important to be aware of the behavior of the outputs should the device perform a firmware reset

• Resets occur:
  • After a firmware update
  • After a parameter change
  • If sent via a command from the server
• If a reset occurs, the output will not persist through the reset - there will be 2 *tiny* 'blips', both less than 1ms
  • 1ms is extremely unlikely to have any impact - but it is something to note if you are using the output to drive a pump/motor etc.

Outputs can also be configured to run for a specific amount of time - i.e. 30 seconds. The timed digital output is not guaranteed and should not be used for critical applications. An unexpected reset may lead to the digital output reverting to its previous state sooner than expected, or the digital output may be set a 2^nd time immediately after the reset"