Going back to high school chem, a liter is 1000 milliliters and if the liquid is water one milliliter weighs one gram. Therefore a liter frequently weighs 1000 grams.
Some folks like to express their test results as grams per liter, roughly grams per 1000 grams. The target for a wine would be 5 to 7 grams of tartaric acid per liter (1000 grams) Playing with the decimal point it is easy to convert said number to 0.5 to 0.7 grams per 100, , , ie as a percentage.
In my case most of what I have read expressed target range as a percentage plus the first TA kit I bought expressed it as a percentage so I started thinking those units. The technical definition ought to be to call the system “volume percent”.
With lab testing a lot of samples would be cooked food product ground up in a Wharing blender. ie a weight based test, in which the resulting answer would be “weight percent”. To complicate units it could be expressed as percent sulfuric acid in European literature, or as percent malic acid if talking with cider folks.