Your placing the acrylic/plexiglass in the 10gal tank for baffles right? If so, regardless of which is correct either would be fine; go with the 1/4" as the 1/8" will be too thin and bow on you. To your question though, I think people interpret this differently and I am not exactly sure which is correct. When I think of plexiglass, I think of safety glass that may be used on a storm door to prevent injuries if someone was to run through it. Plexiglass usually has a blue film paper on it to prevent it from being scratched before used, acrylic has a brown paper.
I know those aren't very definative but my belief and thoughts.