Textiful Credits are used for each outgoing text that is sent. An outgoing text is considered any text that is sent to an individual's phone. This includes the messages that are sent to individuals when collecting email addresses through text messages.
Collecting an email address or any other information requires two outgoing texts - one to ask for the information and another to send the confirmation message
In other words, if you collect 5 email addresses, it would cost 10 credits. If you send a text campaign to those same 5 individuals later on, it would cost 5 credits.