Add New (+) button behavior of sub grid in D365

While working with a sub-grid in D365, a question was thrown regarding behavior of Add record (plus) button. Many of you must have observed that for few sub-grids, click action of Add record button opens a new form to create a new record while for others a new line is added in sub-grid where one can add the record name and associate it. You might have figured the reason for this earlier but it was now that a college helped to figure out why it happens for me.

For those who were still looking for the reason of this behavior like me, this is all related to requirement level of the look-up field on child record. If Look-up is configured as business required then a new form is opened else the grid provides an inline row to associate the record. My assumption for this behavior is that if the look-up is business required, then it is assumed that all the existing records are already associated with some parent record thus user should create a new record. Always correct me if i’m wrong 🙂