The operator is an extension of the IEC 61131-3 standard.
You can use the TYPE name of an enumeration for unique access to an enumeration constant. In this way, you can use the same constant names in different enumerations.
The enumeration name is prepended to the constant name with a dot (.).
<enumeration name>.<constant name>
Example
The constant Blue is a component of both the enumeration Colors and the enumeration Feelings.
color := Colors.Blue; // Access to coomponent Blue in enumeration Colors feeling := Feelings.Blue; // Acces to component Blue in enumeration Feelings