xlsatom, xprop

Hy,
I googled a lot, but w/out success.
Who know how I can set information in the X-Server and get them back
via an Xclient.

xlsatom shows some "ATOMS", How can I set them
xprop has a switch calles -set name value (this is what I need, btw).
But I am unable to find the right syntax.

regards
TAB
.