@Yeshan Santhush Indiketiya Hewage - Thanks for the question and using MS Q&A platform.
The "isOptional" property in the relationshipAttributeDefs of the Purview REST API's "column" type specifies whether the relationship is optional or not. If the value of "isOptional" is set to "false", it means that the relationship is mandatory and must be present whenever you create a Column type entity. On the other hand, if the value of "isOptional" is set to "true", it means that the relationship is optional and may or may not be present when you create a Column type entity.
Regarding your question, it is possible to create entities of the column type without defining the "composeSchema" relationship with the created Column entity because the "isOptional" property only specifies whether the relationship is mandatory or optional. It does not prevent you from creating an entity without the relationship. However, if you try to use the entity without the required relationship, you may encounter errors or unexpected behavior. Therefore, it is recommended to always define the required relationships when creating entities to ensure that they function as intended.
Hope this helps. Do let us know if you any further queries.