Just a quick note here: Scripts or UI policies are not running in the forms on the background, while the checkbox still works when the variables become visible in the back.

For instance, you have an item for user creation and modification. On creation, a first and last name are mandatory. On modification, a reference to the sys_user table field is made mandatory. If creation is selected, the modification fields are removed from vision and vice versa.

The reference field should only be mandatory when dropdown is modification. You have multiple options:
1. Write a client script, that makes the field mandatory onChange/onSubmit.
2. Create a UI policy, that sets the field to mandatory, when the condition is met.
3. Set the checkbox on the variable itself, and make it not-mandatory with Client script/Ui policy when modification is not selected.

Client scripts and UI policies both work only on the front-end (the service catalog form) while the checkbox also works on the back-end. So in situation 3, your field is mandatory on the back-end, even if the dropdown is not set to modification, making it impossible for the handler of the request to make any changes to the forms, unless the field is filled.

Assuming that in most cases, handlers of tickets at the back-end will not be required to fill out the forms' variables, it is often better to use policies or client scripts to make fields mandatory on forms, instead of using the mandatory checkbox.

Regards,

Wesley