Nyloboard - for the material cost ($220x6) alone I could've done 4-plus 27-foot trailers using the MDO panels I found. Sounds like a great product if they didn't price it out of reach for most of us; the factory direct outlet angle sure made that tempting back when it was a smaller premium over Marine Plywoods etc..
Sign Board has a hundred different flavors since its a generic term, there are plastic and foam and masonite cored panels, aluminum foil covers, etc.
What I used can also be called as concrete form panels - its truer label was medium density overlay (MDO) panel, "Signal MDO". Olympic Panel Products - Sign Making
I still haven't seen this stuff age and weather yet, but I have invested the time in 5 coats (3 primer & 2 porch floor) of paint before I install it.
I only found mine after a half-day of googling keywords and calling around and found it 20~ miles away at an old-time lumber yard on the far side of Minneapolis that sells to sign-makers too. You might try contacting local advertising sign makers to see where they purchase from - $42 a 4x8 sheet of true 3/4" (not something-64th) made using this a no-brainer on my part when big-box stores are asking $30-35 for their 3/4-nominal plywoods...