Immutable Variable Address
Alias variables are immutable with regards to their pointer address, thus re-assigning an address will return an error. For example the following code will not compile
FUNCTION main
VAR
foo AT bar : DINT;
bar : DINT;
baz : DINT;
END_VAR
foo := baz; // Valid, because we are changing the pointers dereferenced value
foo REF= baz; // Invalid, `foo` is immutable with regards to it's pointer address
END_FUNCTION