DPI is printer dots; PPI is pixels per inch. For size planning you use pixels ÷ inches — whatever label the software shows.
What stays the same
Your file’s pixel count does not change when you pick a different DPI target. You either print larger with lower density or smaller with higher density.
Practical takeaway
Choose DPI based on viewing distance and criticality of detail — then check whether your pixels support that print size.
FAQ
Which should I use for print planning?
Use pixels and target print size. Treat DPI/PPI as the density you want on paper, not as a magic quality switch.