On the TI-89, a function is a mathematical relationship that takes inputs and produces outputs, allowing for calculations and graphing. In contrast, a program is a sequence of instructions written in TI-Basic or another supported language that can perform complex tasks, automate calculations, or manipulate data. While functions are typically used for single calculations, programs can execute multiple functions and handle more elaborate logic and user interactions.
I believe anything under the level of a TI89 are allowed.
For my ACT, I brought two calculators (make sure not a TI89 for they are not allowed), a water bottle, snacks, sharpened #2 pencils, a big eraser, and my brain.