A boolean type value depicts a choice between 2 possible values: YES and NO. These 2 choices are available through the use of the general variables: $YES and $NO.
You would give a variable a boolean type value as follows:
<cx:let name="b" value="$YES">
... $b contains the value YES ...
Boolean values are used for conditional logic (see element cx:if).
In the way a boolean-value works the value NO equals the so called nil-value and is any other value interpreted as being the value YES. It is therefor also possible to use a value like "1" or "" (an empty string) as alternative for the variable $YES. However, for transparency always use the variables $YES and $NO.