Conditional Field
Show a custom issue field only when another field has a particular value. This example asks for Kernel Version only when Platform is Linux.
Configure the fields
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As an administrator, open Administration > Issue Settings > Fields and add a field named Platform with type Enumeration. Leave Allow Multiple off and add the choices Linux and Windows. Enable Include When Issue is Opened. You can limit Applicable Projects to a test project.

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Add a field named Kernel Version with type Text. Enable Show Conditionally, select Platform, choose has any value of, and select Linux. Enable Include When Issue is Opened for the same projects. Leave Allow Empty Value off to require a kernel version whenever this field is shown.

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Open a new issue in an applicable project. Selecting Linux reveals Kernel Version; saving with it empty reports a validation error. Selecting Windows hides the field and allows saving without a kernel version.

New fields are included in newly opened issues. To add them to existing issues, use the issue list's batch edit operation. If the workflow reconciliation notice appears, follow its link after completing your changes.