offset sealer?????

Halo!

Anybody knows what ''offset sealer'' is?

First i thought it was an elegant way to say varnish,but then lower in the job specs, varnish was described as ''varnish'' so the sealer must be something else!

Have a look if you please...

Finished size: 21x29,7
Text: 152 pages printed 4+4 colors + offset sealer
Matt paper 115 gs
Cover: 4 pages 4+4 colors + offset varnish
Matt paper 200 gsm
Finishing: sew binding

So if anyone has an idea,please let us know!