sed: what does this do

[gold]

Can someone tell me what this code below do?

sed -e 's;%VSN%;$(_VSN);' $? > $@

[rbo83]

It replaces the first occurrence of %VSN% in the input '$?' with the value or the '_VSN' environment variable and writes the result to the output '$@'

$? is the result (possibly return code) of a previous command in the script
$@ is the list of all the positional parameters on the script call

Here is a tutorial on special linux variables :
http://linuxreviews.org/beginner/abs.../en/x1790.html