Where Is It?
The Holds block is located in the exceptions drawer of the Blawx toolbox.
What Does It Do?
The Holds block is used to query whether a conclusion was reached according to a given section of the rules, and that conclusion was not overruled.
The Holds block is a statement block which can be stacked with other statements. It accepts an input value that can be a section selector block or a variable, and an internal statement stack, which should be a single statement block, which is the conclusion.
It is not usually necessary for you to use the "holds" block explicitly. If you use attributed rules, "holds" statements are made automatically for you. And if you as a question about an attribute, the section that held that statement will be included in the explanation. The only time you will usually need to use the holds block is if you are troubleshooting problems with your defaults and exceptions.