中信免麵簽 question

My parents went to the states last August for my wedding. They stayed here for
only a month.

Now I would like to invite them again. I was told that if they use 中信免麵簽,
they DON"T NEED
1) financial support I-134 form
2) invitation letter for the visa officer.

I am not confused. Last time they came here for my wedding, and this time for
visiting. They got different reasons to come, why no need for invitation
letter??? Anybody knows why?

Thanks,