What is the difference between DPI and PPI?
PPI describes image pixels per physical inch; printer DPI describes output dots. They are related to print production but should not be used as interchangeable image-resolution measurements.
PPI describes image sampling; printer DPI describes output dots
If a 3000-pixel image is printed 10 inches wide, its effective image resolution is 300 PPI. A printer may place many more ink or toner dots per inch to reproduce each image pixel. Changing a printer's advertised DPI number does not create missing image pixels.
Use the numbers from your actual job
The guide explains the decision; the linked calculator gives you the value for your own file or production assumptions.
PPI describes source image sampling
Pixels per inch compares digital image pixels with a physical output length. A 3000-pixel-wide image printed 10 inches wide has an effective resolution of 300 PPI.
DPI describes printer output dots
Printer dots are device marks used to reproduce tones and colors. A printer can use multiple output dots to represent one image pixel, so an advertised device DPI is not a substitute for source-image PPI.
Use PPI when asking whether an image is large enough
For print-size planning, divide available source pixels by final print inches. That is the number that tells you how much raster detail is being supplied per inch.
Do not fix a low-resolution image by editing metadata alone
Changing a metadata resolution field without adding real pixels only changes the interpreted physical size. It does not restore detail that was never present in the source.
Practical answer
PPI describes image pixels per physical inch; printer DPI describes output dots. They are related to print production but should not be used as interchangeable image-resolution measurements.
When the decision affects a paid print run, book upload, client order or expensive material, keep the calculation with the job notes and verify any external specification again immediately before production.