Select Conditions is the place to simply "gather" the devices you'll want in the Rule. In the simple cases, RM "guesses" what you want the rule to be, and automagically fills that in. When you have two or more Conditions defined, RM can't just guess if you want them ANDed or ORed. The Define Rule section is for that. You pick a condition and then AND OR (or parenthesis) to create your intent.
Conditions are what is going to be the activator for the rule and what capabilities that activation element has.
Define Rule is where you specify which of those Conditions (the activation element and its capability) are going to cause the rule to become either True or False
If you have two or more Conditions joined by the operator AND, both of those Conditions must match. If you join two or more Conditions with the OR operator, then either of those Conditions or matching sets of Conditions can trigger separate results based upon either the rule truth, or falsehood.