Googling 'HDPLA' has so far availed me very little.
http://3dinsider.com/what-is-pla/ indicates that /all/ modern PLA is 'high density' compared to 'the early days'. But a fellow at the local makerspace indicated that he was specifically using 'HDPLA', with [he claimed] markedly better properties than regular PLA in the final product.
A company named Florion claims to have some secret sauce they add to their PLA, and Maker Filament touts a high temp PLA; but neither seems to use the 'HD' prefix.
I haven't yet been able to track down the fellow to get any more specifics from him. There's no reel of the stuff sitting around so I can't look at its labeling for clues. He claimed to be fabbing lab fittings but I don't know the intended operating situation/requirements.
It's possible he meant something else, like HDPE, which would be quite inert and thus a good choice for lab fittings--but I'd like to think that if it was lab equipment he was making he'd get the name of the polymer right.
Comments, including any regarding the Florion or Maker Filament or any other 'high performance' PLAs, would be most welcome.
I doubt that it means very much at all. Filament manufacturers are very tight-lipped about the co-polymers that they add to their base stock in order to improve handling and performance characteristics, so it is impossible to say. The only common attribute that I can see is an advertised diameter tolerance of ±0.02mm. Maybe HD stands for high-definition, rather than high-density?