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To create a reusable variable, e.g. for a list of user IDs, first set up your targeting logic as normal for one flag, adding a rule that uses this list of IDs.
Then open the context menu on the list of IDs and convert it to a variable.
The list will be replaced with a reference to the new variable.
You can navigate to the variable to view and edit this list.
And you can reference this variable when adding rules to any of your other flags.