Either true (1) or false (0). Variables of this type should not be used to store any other values. Like C++, 20-sim evaluates non-zero values as true; only the value of zero is evaluated as false. Normally, the value of one is used to indicate true.
boolean b = true;
boolean c = false;
boolean D[2,2] = [true,true;false;false];
a = b and c;
When used in standard functions, booleans are treated as reals with value 0 or 1. An equation like:
boolean a = true;
c = sin(a);
would yield c equal to 0.8415. Try to avoid these constructions, since they lead to confusions.