A specific plan tier, like 50 contacts or 100 contacts etc., allows you to communicate to that many unique contacts monthly.

For example: 

You can use a premium-75 plan to communicate freely with up to 75 unique contacts monthly. You cannot use a premium-75 plan to communicate to 200 different contacts in a single month. You would need a plan tier of 200 to do that and it would require an upgrade from the 75-contact tier to the 200-contact tier.
Please ensure your contact lists are edited to your liking before sending your first message each monthly cycle to avoid having any unwanted contacts applied towards your current monthly allotment.

More details:
Your plan has an allotted tier for how many contacts you may communicate to during each month. For example, someone on the premium-75 plan can communicate to up to 75 unique contacts during the month.

Once you send a communication each of the contacts in your account is applied towards your monthly allotment (even those who may not have been sent that particular message) as we will reserve resources for your entire list at this point. A single unique contact is only applied once per month to the monthly allotment no matter how often they are communicated to.

Once a contact is counted towards your monthly allotment, deleting or deactivating the contact does not remove them from the current month’s allotment count. Deleting them now will keep them from being added to your next month’s allotment however.  
If you wish to add additional contacts beyond your monthly allotment you will be required to upgrade to the correct plan tier at the time of sending.

Hypothetical scenario:
A user on the premium-75 plan has 75 unique contacts in their account and sends a communication to 50 of those contacts with their first message of a new monthly cycle. All 75 unique contacts in the account will be applied towards the monthly allotment.

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