Typically speaking, you'll want to leave the type as Normal for all standard recipients; the other types are used for special configurations.
Ad-Hoc recipients function as pass-through for provided input. Put another way, an ad-hoc recipient lets you pass in the phone number or contact information you want the message delivered to rather than having that phone number exist as a static entry in the recipients list.
If you have PageGate's GetMail API configured to host an email domain, this recipient type tells PageGate that this recipient is not a messaging recipient and cannot be messaged by the connectors. Instead, this recipient exists solely as a hosted mailbox that provides an email address that can be checked by POP.
This type has nothing to do with the delivery of SMTP/Email and should only be used when this recipient should solely exist as a hosted mailbox.
If you have PageGate's GetMail API configured to receive email traffic, this recipient type tells PageGate that traffic sent to this recipient will contain contact information in the subject line.
For example, let's say you were hosting the sub-domain messaging.something.com and that you have a recipient named 'alerts' that has been configured as a Multi-Page recipient. You could then send an email to email@example.com with a series of 5 recipient names in the subject line and PageGate would send the body of the message to all recipients and groups referenced in the subject.