When a case enters or exits a pool the actions run based on the 'Execution Conditions'.
'Execution Conditions' look at data values stored in the case and if the rules match the action will be executed. If no Execution Conditions are present the action will run.
In the below example, the system will send an email if the case value 'approved' is equal to 'Yes' (true).
The first section of execution conditions are AND checks and all must match to continue onto the second section. In the second section, each entry is OR'd and if any match (and the first section results in a true) then the action will be executed, otherwise we will move into the next action in the list.