The operator is an extension of the IEC 61131-3 standard.

The ‎AND_THEN‎ operator is permitted only for programming in structured text with the ‎AND‎ operation of ‎BOOL‎ and ‎BIT‎ operands with short-circuit evaluation. This means:

When all operands yield ‎TRUE‎, the result of the operands also yield ‎TRUE‎; otherwise ‎ FALSE‎.

However, CODESYS also executes the expressions on other operands only if the first operand of the ‎AND_THEN‎ operator is ‎TRUE‎. This can prevent problems with null pointers, for example in conditions such as ‎IF (ptr <> 0 AND_THEN ptr^ = 99) THEN...‎.

In contrast, CODESYS always evaluates all operands when using the ‎AND‎ IEC operator.