Hello, thank you for your query.
Doctrine of occupied field thoroughly refers to those legislative entries of state List, which are expressly made 'subject' to a corresponding entry either in the Union List or the Concurrent List.
This doctrine is only concerned with the existence of legislative power.
Under Article 254 of the Indian Constitution, as soon as a union law receives assent of the President, it is said to be a law made by the parliament. The actual commencement of the law is not important for the purpose of application of Occupied Field.
In the case of State of Kerala vs. Mar Appraem Kuri, the Centre enacted the Chit Funds Act (Central Act). Central government would have to issue a notification under Section 3 of the Central Act if law was to become operative in any State. State of Kerala enacted a separate act. However, the Central Act did not get notified in Kerala resulting in a situation wherein there was only one Act in force-the Kerala Citties Act. Supreme Court held that even un-notifed Central law attracts article 254.
I hope this helps you.