TPipe /dup=...format spec

TPIPE - Text filtering, search, and substitution
format - %[index ":"][-][width][.precision]type

I haven't had any luck with specifying the format.
What are the choices for the [index":"] part? Are the quotes required? Is the 'index' required? What does it do?
I'm thinking the rest is standard printf stuff, eg ...-19.19s, 6.2f, ...