​You need to have a Program and make sure you have set either Per user login code or Per user phone as the authentification method for the Program. This needs to be done in order to identify each user uniquely.
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Enable register check-in code if the participants are going to enter their own wristband number (RFID check-in code) in the program
(e.g. if they are given identification wristbands that will be used at check-in-points). When enabled a new 'check-in' page will be displayed in the program menu.
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1. Page heading
​Set the heading of the page.

2. Page menu text
​Set the text displayed for the tab in Program. (Max 12 characters)

3. Page intro text
​Set an introduction text letting your users know what to do.
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​4. Allow changes to check-in code  
​By default, participants are only allowed to register a check-in code once. Enable to allow changes.

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If you want the registered check-in codes to be validated against a list of valid codes, you can read this article.