You can create a Multi-Level Cascade Select custom field for your issues, by following these simple steps:
Go to Settings → issues.
Go to Custom field → Create custom field.
Find Multi-Level Cascade Select (multiple selects) in the Advanced list.
Set a name and, optionally, a description for the new field.
Choose the screens and contexts where you want to associate the Multi-Level Cascade Select custom field.
MLCS is now ready to use.