Documentation
Statistic Values in Smart Rules
How to use calculated statistic values in Equation and Formula Smart Rules
Currently it is possible to use calculated statistic value in Equation and Formula Smart Rules.
In Variables section, select device and pick one of its predefined statistic values:

Equation: Turn on vent if humidity in bathroom is 10% higher than in other room
Variables:
Hu Humidity in bathroom - Average by hours
Hu1 Humidity in other room - Actual value
Equation:
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Action:
Vent → Turn on