Hi @tahnalos 

Check 

Configuring read-only security options 

The Instance Configured read-only option maintains backwards compatibility for read-only fields created prior to the Australia release, and also enables you to test other read-only options on a non-production instance before implementing them on your production instance.

When the Read only option is set to Instance Configured, read-only behavior is determined by the glide.read_only.legacy_read_only_behavior system property. The default value for the glide.read_only.legacy_read_only_behavior system property is client_script_modifiable, which means the field appears as read-only in the UI, but can still be changed by a client script.

You can test read-only behavior on a non-production instance by updating the system property with the following values and then validating that your fields and client scripts work as expected.

• display_read_only
• client_script_modifiable
• strict_read_only