I think it is client-side. The user would be filling in the form and type in the item number, then when they tab off, I need it to validate that item number by going to another table and finding that item number in the column and then looking to the next column to see if it can be deleted or not, then go back to the user and either let them continue with the form or show an error message.
If you need this to be done via client-side you should be looking at adding JavaScript to your page/form
If you need this to be validated via Server-side you should be looking at Dataverse plug-ins