I was introduced to this brand when I first visited the Philippines in 1968. This is the original that Rufina de Lucas developed in 1900 and created an empire. It is also slightly different from other Southeast Asian fish sauces. For one thing it contains less sodium. This contains 570mg of sodium per 5 grams versus 4450mg for a typical Thai fish sauce like Thai Kitchen Fish Sauce.
Another subtle difference between this and other Southeast Asian variations on fish sauce is this is slightly less pungent - it has a smoother flavor that blends perfectly into cooking as well as as a condiment. I use it as a salt substitute when cooking sinigang, and as a condiment on vegetables. It transforms eggplant dishes like inihaw na talong or rellenong talong into culinary delights. I also use it on some types of fish such as salmon to enhance the flavor.
Because this is less salty and pungent than other types of fish sauce I use it to make dipping sauces as well. One such sauce is two tablespoons of this patis, the juice from a green kalamansi and chopped siling labuyo. When I cannot get kalamansi I substitute key line, and for siling labuyo I substitute bird's eye chili peppers.
You can come close to capturing the flavor of Rufina patis with some premium brands of Thai fish sauce, but you will not enjoy the true flavor of the Philippines. For that this is the only brand that has that magic balance of saltiness and flavor.