You can be sure that the USP Glycerin from Walmart, however it is labeled, is good USP Glycerin. Even if it doesn't say Humco on it it was probably packaged by Humco, in Texarkana.
There are people who have talked to there friend at the Humco plant, a Quality Manager, or maybe it was a Marketing manager, or dock worker, that has said that their glycerin is Vegetable Glycerin. Even though they don't put that on the bottle, and recommend it for skin care (like baby butts), it is pure enough to be called USP. That is good enough to vape.
Some folks have complained about getting a quezy feeling or upset stomack from
vaping Walmart Glycerin (I did). and better results from USP Glycerin that says on the label that it was made from vegetable sources. But, what do we know. Chemists claim that the molecules in regular glycerin from biodiesel runoff, or glycerin made from animal fats, or even synthetic glycerin made from left over petrochemicals are exactly the same as Vegetable Glycerin made from only vegetable sources. The Chemists know their stuff. USP Grade Glycerin is all the same, chemically.
The small amount of impurities in glycerin may be leftovers from the original process such as hydrocarbon cracking, contaminated pig feed, or even fertilizer for the palm trees.
Walmart is a trusted source. Their glycerin is just as good as any supplier's even though no one wants to commit to it being sourced from vegetable material (like palm oil) and they recommend it for skin care, not even food use. Buying a liter or even a gallon of USP glycerin, that happens to be Vegetable Glycerin, and happens to even be Kosher from an online source isn't as much fun as going to Walmart to get your vaping supplies and sure isn't as cheap as Walmart.