I don’t think its ever appropriate to ask for money for your wedding, and even less so to ask for “donations” for the wedding PLANNING, as opposed to your life after the wedding. Its okay if you let people know AFTER they ask you DIRECTLY what you want to say “we don’t really need anything but money would be wonderful” but asking is not appropriate. However, I think this applies to non-moneytary gifts and regstries, too–you can’t announce your requests, you can only make them known after asked or discreetly through the grapevine of family and friends.
Additionally, I don’t think you should plan a wedding you cannot pay for. I would much rather give you a gift for your LIFE together, not for one DAY.
To answer your questions directly:
* Are there any written rules that determines the type of gift a couple is allow to ask for?
No there are no written rules. However, all versions of etiquette of which I am aware, whether listed in books, taught in a formal class, or handed down from parent to child, include something along the lines of “it is never appropriate to ASK for a gift of any kind. Gifts are always optional, and to ask presumes that you expect to receive one.”
* What exactly does gift giving really mean?
Gift giving is a means for people to show their love, friendship, appreciation, or congratulations to other people–family, friends, associates, etc.
* What is the difference between a wedding registry / gift certificate and a donation to help a couple with their planning expenses?
Assuming the gift-giver is freely making the choice, not much. But many people, myself included, feel that a wedding present should be something to celebrate your NEW life together, however that may be. A gift of money “for the wedding planning” would not meet that goal. A gift of money “for the new house, or honeymoon” is different. Also note that you donate to charity. Money given to you for your wedding, whether it be for the planning or for the life after, is a gift, not a donation.