When you select or create a filter for a marketing campaign, you may notice that the number of Total Available Records doesn't match the number of records listed in the filter you selected. This is either because:
The records are already on another marketing campaign. A record can only be in one marketing campaign at a time.
The records completed another campaign and are within the Delay records before entering a campaign period.
The records don't have a primary contact with at least a primary mailing address, a primary phone number, or a primary email address. These records cannot be used for marketing purposes, as there is no available information to market to.