The monthly plans are for those who own a base of contacts and wish to send emails to the same contacts several times a month. Let's say you signed for a plan of 1000 contacts with unlimited emails. In this case you will only be able to enter up to 1000 contacts in the emailmanager system over a period of 30 days. If you exclude any of your contacts, you will only be able to insert new contacts at the beginning of a new period.
The pay as you go plans are for those who do not own fixed number of contacts (the number of contacts that you enter and/or delete from the system does not matter). Let's say that you have signed for a plan of 5000 emails during the period of 30 days and you have a list of 2,500 contacts. In this case, you will be able to send 2 email campaigns to your contacts.