Whats the foam tape I saw on a site recently?

[milkman]

On a recent job I saw some dense grey foam tape/sheet being used in the cavities around door openings.
Thing is it'd be really useful for tool trays and bags. Can anyone tell me what its called?
Also out of curiosity whats it for?
I don't know if it was being used correctly where I saw it or whether it was there cos the client was a bit nuts about dirt getting on the cavity insulation.

Thanks
Marko

 

[Deluks]

Cavity closer?

 

[^woody^]

A sheet of foam?