SuiteFlow (Workflow Manager) Reference: Before User Submit Trigger
The Before User Submit trigger is a Client Trigger that fires when the Save button is clicked on the form before submitting the record. This trigger is similar to the Before Record Submit server trigger.
If an error occurs during this trigger, a pop-up message will show up that describes the error and the record will not be submitted.
Use this trigger as an alternative to Before Record Submit where running into an error would cause problems and inconvenience. For instance, a custom error message triggered on Before Record Submit will consequently result in the user having to re-enter data again on the form, while on Before User Submit, a pop-up error message is displayed instead that can be dismissed without losing any information entered.