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Adding the 'User Is In Custom Field' condition check to a workflow transition means that a user cannot see that transition unless they are (or are not) in the specified custom fields, (typically a multi select/user picker custom field) Note, this condition check only applies to custom fields.
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You must specify the field that is the prerequisite for this transition, and whether the user must be in the custom field or not, when configuring the conditioncheck. For example:
For information on how to configure a condition check for a workflow transition in JIRA, see the JIRA documentation.
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A workflow is configured so that the 'Close' transition has the 'User Is In Custom Field' condition check where the user must be in the 'User Select' field. A user viewing an issue in that workflow will not see the 'Close' transition unless their username has been selected in the 'User Select' field.
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