Tan Tui is a set.
There are many variations. 10, 12, 14 and 28 road versions exist (probably many variations on those numbers, themes and methods as well). Most are done in a straight line, back and forth, over and over. Each 'road' is one of the lines of the form.
As a set, it is practiced inclusive in other systems. I have never heard of any 'systematic' approach to teaching Tan Tui as a stand alone system. That doesnt mean I am right in that regard, I allow that the world is big enough to include things I have not seen. I speak only of my own knowledge in that regard.
That said, it is a set rich in content and training application. The stance transitions as well as the vigorous use of the arms in conjunction with moving, striking with arms/fists and kicking are a great workout.
I practice a 12 road Tan Tui and certainly can attest to it being a good workout, developing strong shoulders, waist and legs. Posture is important in the form, so certainly it offers a great foundation for other systems, and is often used as an introductory set for TCMA styles.
Best of luck in your search. You can also search this and other forums for Ethnically Muslim origin martial arts and gain some insight into the training methodologies used.
Rob