MediaWiki:Centralnotice-impression-diet-identifier-help/en

An identifier for persistent client-side data stored by this feature (such as impression counts). This field is optional for most campaigns. If you leave it blank, campaigns in the same category will share impression counts, or, if this campaign does not use the category feature, counts will be specific to this campaign. To share this data across several campaign categories, specify the same identifier for each campaign. Note: certain campaign categories are configured to use cookies for this feature. For campaigns in those categories, this identifier will be the cookie name, and is not optional. Also note that if the identifier is changed during a campaign, all counts to date will be lost.