This convention is also used in Smalltalk, where only the special objects and can be used in a boolean expression.
Executing one of two subprograms according to the value of a boolean expression (selection)
Executing a subprogram until a boolean expression is true (iteration)
The boolean expression must be a compile-time constant value, for example would be a valid expression in that context.
TEX did not provide and or or connecters to make more complex boolean expressions.
A conditional is either a (boolean) expression and a process (b P) or an IF construct.
These terms can be used to write a minimal boolean expression representing the required logic.
The point is simply classified against all the underlying primitives and the resulting boolean expression is evaluated.
Using the for statement and a boolean expression:
Conditionally show or hide an element, depending on the value of a boolean expression.