Sub-Grids are an excellent way of displaying data related a record directly on the record form. In Dynamics CRM 2011, there were clear “Add New” and “Add Existing” buttons for each sub-grid so a user could determine how they wanted to associate something to the current record. With Dynamics CRM 2013, these buttons were both rolled into the singular “Add” button in the corner of each sub-grid:
This has caused confusion among legacy and new users alike, as the button does not have a consistent action across related records.
CRM with John Underwood 