Email credits are simply how many emails you can send from your confetti account. Every credit is one email.
The only messages that count towards your credits are the ones you send out manually, the ones generated by the visitors clicking the apply button for example do not count.
So, when you invite someone, that is one credit.
When they click the RSVP button (or by a ticket) and get a welcome email that is not a credit.
If you send out a reminder to them before the party, that is one credit.
If they realise they can not come after all and change their status to "not coming" so they get the "decline" email, that is not a credit.
Please note that if you change their status from waiting list to attending or from attending to declined the resulting email will count towards your credits.
Basically the rule is: if you generate the email is counts, if your guests generate the email it doesn't.