Registrations is what the host purchased.  Mapped Registrations is what option the registrant is mapped to.

If the host is not attending, they will not have any mapped registrations since they are not actually attending, so they aren't mapped to a certain registration option.  If the host is not attending but bought tickets for others, those people attending will have mapped registrations.  The host will have registration options though--this is what he purchased, whether or not he is attending.

Registration options should be used when you want information about what the host purchased, regardless of registrant information or attendance.

Mapped registrations is what should be used when you want information about who is actually attending and what option they have.