You can think of Blawx has having two different categories of statement connectors: asking, and telling.
A rule "asks" whether the conditions are true and "tells" that the conclusion is true. A question "asks." A fact "tells".
What a block means can change slightly depending on whether you are using it in an "ask", or a "tell", but the differences are usually pretty intuitive.
What might be less intuitive is that the blocks that you use to create objects and categories can also be used inside an "ask." So a block reading "bob is a person", when used in a "tell", creates the fact that bob is a person. The same block, used in an "ask", checks to see whether that fact has been created somewhere else.
Many blocks that are typically only used inside a fact can also be used inside the conclusion of a rule. For example, a rule might conclude that "bob is a person", creating that object only when the conditions of the rule are true.
Note that right now Blawx cannot differentiate between declarations used in an "ask", and declarations used in a "tell". So if you use "bob is a person" in the conditions of a rule, Blawx will nonetheless include "bob" as a "known object".