trans maps are used to erase all or part of something. Assuming poser since it is in this thread, in the material room you add it to the square which says transparencies.
Since your handle is rifleman...
If you have a UV mapped rifle (a rifle with a template file), you can load the template in your paint program. You paint the areas white that you want to keep, black that you want to erase or gray if you want a ghosty effect.. since a ghost rifle would be silly.... you select the rifle, apply the transmap you made which has a black for most of the barrel, black for most of the rear stock and voila you just turned your rifle in to a pistol.
This can be especially useful for complex objects with independent textures for the different parts. For example, the crown I made... you want a plain one with no red velvet inner hat, no gems. In this case you load a all black image and apply it to those parts which are to disappear completely. And you are left with a plain gold crown.
Suppose you find a long robe for PDF and you wanna make it a bit racy.... you can use transmaps to make it a miniskirt (erase the lower portions, erase the sleeves, create a plunging neckline.... you could even load some skin tight pants underneath and with a brown basic color and a medium gray transmap she has pantyhose.... or with a checkered transmap she is in fishnet stockings.