a good amount of the twl sdk is exactly the same as the nitro sdk aside from some adjustments to the card and filesystem libraries. there are differences in other libraries though, i occasionally have to make up a name for a function. the "i" functions are internal to the devkit only, not for use by app developers. they're not dsi-related. internal functions in the nitro system, wifi system, sdk, and other libraries are marked with the library name and then an "i" are internal.
btw, just for the sake of trivia, the entire set of modelling programs usable with the nitro system are 3dsmax, maya, si3d, and xsi. some of the stuff i thought was different modelling programs are actually general-purpose for the listed programs.