The attribute selector block is located in the Known Attributes drawer of the Blawx toolbox.
The attribute selector block is used to set or test a value for an attribute of a given object.
The attribute selector block's appearance can vary due to the attribute display block.
It always has two internal connectors, one for the object, and one for the value. The object connector accepts an object or a variable. The value connector accepts a variable, or a value of the correct type if the attribute has a datatype, or an object, if the attribute's datatype is a category.
Most of the work that you will do setting out rules in Blawx will be done using attribute selector blocks and variables.