What does it do?

EDIT by DOS386 : moved from Main to IDE Develoment

it exports some data out of the assembling process.
a lot of data,
for example where a label in the outputfile is.
there are some threads here in the Forum about the data which are exportet.
and some tools to examine the exportet data data.

Edit:
if you download fasmw package you can find in tools a readme.