Transparency maps are not transparency even are related.
A transparent object has a fixed value on how much it is transparent, a transparency map is a RGB image that modulates this transparency value, so it can turn the object more or less transparent by pixel value.
Strictly alpha maps are not the same as transparency maps, alpha maps only modulate the overall transparency for all colors. A transparent map can modulate each RGB component.
Poser only use alpha maps.

Stupidity also evolves!