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You decide to install the Federation for Jira app only on Jira 1. You can now use functionality of Federation for Jira, such as TODO link on /wiki/spaces/FED/pages/12386528 on a dashboard of Jira 1. This will provide you search results across your whole federation. The results displayed on the dashboard of Jira 1, will include issues from all your 3 Jira instances: Jira 1, Jira 2 and Jira 3.
However since you have not installed the Federation for Jira app on Jira 2 and Jira 3, you cannot use the TODO link on the /wiki/spaces/FED/pages/12386528 on a dashboard of those instances.
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