How does the Supply Planning generates a planned order if no sourcing rule exists for an item?

Summary:

Content (required):

We have observed that in case if the item is MRP Planned and there is no sourcing rule defined for the the item still the Supply Plan suggests a planned buy order for the item with source organization and Suppliers details Null?

IS there any possibility to generated exceptions incase if no sourcing rule exists for the item in the supplies and demand table?

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):