A SEN identifies the product you are talking and helps us know what support you need and if it's still available. A valid SEN is always needed, for support purposes about our products.
If you are using on the Server, you are able to find this by going to Jira Administration → Add-ons → Manage-Addons and click over the product:
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On the Cloud, you need to go to Settings → Apps → Manage apps and click over the product:
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