What is the Issue?
When you click on + Button on subgrid ,sometimes it opens new record and sometimes behaves like add existing by opening search box for existing records.
The Reason –
This behaviour of + button depends on the association between the entities. Adding new record will strictly associates records and add existing will allow you to associate records if required.
To change the behaviour of the + button you need to change requirement level of the lookup field of the entity on which you have subgrid i.e. parent entity.
For example- you have 2 entities – AEntity and BEntity.
AEntity contains subgrid of BEntity.
On BEntity form we have lookup field- AEntityLookup
Now behaviour will be decided on requirement level of AEntityLookup field.
1. If requirement level : Business Required
Then it will behave as Add new record of BEntity.
This will also change behaviour of record level delete button as record delete.
2. If requirement level : optional/business recommended
Then it will behave as Add Existing record of BEntity.
This will also change behaviour of record level delete button as record remove.