As far as I know, they are the same thing. Here's what I wrote about Undecenoic Acid in my book Candida Crusher:
"Undecylenic acid is a mono-unsaturated fatty acid that comes from the castor bean. It has proven to be a most effective antifungal agent and is the active ingredient in many over-the-counter antifungal preparations.
Just because something is difficult to use doesn’t mean it should be avoided, sometimes you just have to learn a work-around, and I’ve discovered by studying and trial and error that there is an excellent work around with undecylenic acid.
Even though the antifungal activity of undecylenic acid has shown to be more potent than caprylic acid, the problem is that this mono-unsaturated fatty acid is much more sensitive to pH imbalances commonly found in those with candida. Many people with candida have shown to have alkaline pH issues with their small intestine in particular, directly due to the yeast infection and dysbiosis. Many practitioners who recommend different formulations of undecenoic acid, whether it is the oil found in gel caps or in a powder (in case of its salts), are not even aware that there are potentially serious delivery issues with undecylenic acid, it is more difficult to use most effectively than caprylic acid with candida. That is, I find it easier to get a good result with caprylic acid than with undecylenic acid but prefer the combined action of both. All you need to do is to add a good quality digestive enzyme along with it or include a little betaine HCL with the supplement itself.
Let me explain, unlike caprylic acid, studies have shown that undecylenic acid works best in a predominantly acid environment. To ensure that undecylenic acid works best in the small bowel, I always recommend a small amount of digestive enzyme like betaine hydrochloride. This ensures that the undecylenic acid will work exactly where you want it to, in the duodenum and ileum, the two parts of the small bowel mostly effected by candida, thus avoiding the release of excessive bicarbonate (an alkaline solution) produced by the pancreas which would potentially render this fatty acid useless. Many people with candida have multiple issues with their digestive system, including their stomach, pancreas, small and large intestine. As the yeast infection subsides, the digestion slowly but surely improves in health.
Incidentally, fatty acids tend to work better if you take between one teaspoon up to one dessertspoon of extra-virgin olive oil daily as well.