yeah, png files exported from P7X import to APS with no alpha channel, but transparent. 
tiff files exported from P7X import to APS with an alpha channel, usually mostly black.
a solid white alpha channel may mean there's no empty space behind the objects, but
ISTR there was a bug in earlier versions, where some users had an inverted channel.
in a tiff file exported from poser, an alpha channel  that's mostly white, with some grey,
may be the result of activating the depth cue.