It's their new (since January 2013 !) acrylic gloss varnish, very like Johnson's 'Klear' / 'Future', in as much as it's very thin, crystal clear, where a little goes a long way, and is 'self-leveling', and can be used for 'dipping' clear parts as well.
I only found out about it a couple of weeks ago, so checked it out on the Humbrol web-site, as I'd previously thought it was just a larger bottle of the old enamel gloss varnish. From what I understand from the video on the web site, unlike 'Klear' / 'Future', it is a 'proper' acrylic varnish formulated for modelling use, with UV protection to prevent 'yellowing' with age.
It can be applied by brush, or by airbrush without thinning, and I presume that, with the addition of something like Tamiya Flat Base or similar, it can be mixed to create anything from semi-gloss to a full matt, which I'll test in the next day or so.
However, the web-site also shows a satin and a matt version, labelled as 'Coming soon'.
I've done a very brief test to seal the decals on the gloss white wing of the Starfighter, applied by brush, and it looks OK, although it takes longer to fully dry (about 30 minutes or more) than 'Future'.
I've yet to test it on the 'bare metal' finish, but I'll only be sealing around the decals in these areas, and not coating all of the 'metal' finish.