Explain portable apps and when to use them

Task: Explain what a portable app is and when it is a better choice than a normal install for my situation. Context: I use [one PC, several PCs or shared or locked-down PCs]. I want to carry or run [the kind of tools I would want portable]. My storage is [USB stick, external drive or cloud folder] with [size] free. I am [new or experienced] with installing software. Rules: - Explain the idea in under 120 words, then give three situations where portable fits me and three where a normal install is better. - Cover where the settings and files end up, how updates work, and what happens if the drive is lost, without stating how a specific app behaves. - Give a six-item checklist to run before trusting a portable app, including the source of the download and how to keep a backup. - Remind me to respect the rules of any computer I do not own, and to check them before running anything there. Result: The explanation, the two lists of three situations, and the checklist, at most 300 words in total.