The unnamed variable block is located in the Variables drawer of the Blawx toolbox.
The unnamed variable block is used when you need a variable, but you do not need to use the variable more than once in the same scope, and you don't care what object was bound to that variable.
For instance, you might have a rule that makes conclusions about an attribute called "has_a_sibling". In that rule, you might use an attribute called "sibling_of" to check to see if someone has a relationship to at least one sibling, but you don't care who the sibling is. In that case you might write a condition using the unnamed variable, which reads "(Person) is the sibling of (any)".
The unnamed variable block accepts no inputs, and has no fields.
It can be used anywhere an object or a value is expected.
It's a good idea to start with named variables, and switch to unnamed variables only when you are sure your rules are working as anticipated.