LDS dictates the type of underware that members are to wear?
Yes. Find it on google.
LDS undergarments.
The Mormon holy underwear is for a temple ceremony.
Haven’t finished the thread, so this already may have been posted, but I googled and found this link, which not only contains a link to an image of a man and a woman wearing the garments, but also a lenghty and thorough description of the Temple and the ceremonies/rituals. Very informative, I think you will find it interesting! (I know I did!)
http://home.teleport.com/~packham/temples.htm