Behaviours give you more control over fields in Jira. A field configuration customizes how fields behave, based on the issue operation screen they appear on. However, a behaviour in ScriptRunner allows you to take that field customization further, defining how fields behave for issues in a given project or issue context.
You can create behaviours to:
Make a field mandatory depending on other data entered on the issue screen.
Make a field read-only dependent on user role or group.
Conduct server-side validation of field data, before the issue screen is submitted.
Set a field value depending on other issue screen data.
For example, you give users the option to select a checkbox when they do not know their SEN (Support Entitlement Number). You can set up a behaviour to show additional fields when this checkbox is selected, to collect the information required to identify them.
Be sure to read through the Behaviour Limitations documentation before using a behaviour, or if you’re having problems getting your behaviour to work.