gimp - DPI Vs. PPI Vs. Image dimension, what should be taken to consideration?

I've been really facing this problem and I have found a similar link, that does not really address the problem. First of all, I'd like to mention that I use blender and GIMP for most graphics that I design and please read all the way through.

Blender has export option that are only available in dimensions (e.g. 1920 X 1860), and no matter how big we set this up and check the PPI in GIMP(or photoshop, knowing GIMP does only handle 8 - bit images), the PPI is always 72.009.

On employment site, we are requested to provide information if we are ready to supply to our clients a high resolution image atleast a 300DPI, and the dimension is not mentioned.

Furthermore, I have visited many websites and confirmed that the terms DPI and PPI are used quite interchangeably.

Even though we can set the image PPI in GIMP from 72.009 to 300 as easily as image -> scale -> 72 to 300, there is no realtime feedback in the screen to know that the image PPI has been changed.

So, the problem is around what should the output would be, would it be a higher dimensioned image, a higher PPI image or a higher DPI image that would really mean HIGH RESOLUTION?
How can we really set it to a standard 300DPI image, on the aforementioned link, I don't really know if it is not to be optimized for printing.

Answer

Image size (in pixels), print size (in inches), and print definition (in PPI) are completely related by a simple formula:

print_size = size_in_pixels / print_definition

This formula can also be expressed as:

size_in_pixels = print_size * print_definition

or

print_definition = size_in_pixels / print_size

But it should be clear that you can only change two terms at the same time, the third term will completely determined by the other two. For instance a 3000x1500 pixels image printed at 300DPI yields a 10"x5" image.

When you create an image in Gimp, the image creation dialog lets you give the image size in inches (just change the units), and specify a print definition. Gimp will compute the required size in pixels for you.

Good quality printing requires at least 300PPI.

Print definition, as associated to the image, is usually only indicative (unless the image has been scanned, in which case it allows you to print the image at its native size). You can change it using Image>Print size. When it is not specified, the universal default is 72DPI.

DPI and PPI are about the same thing on screens and monochrome laser printers. On color printers, a pixel can be implemented with very many dots of different colors, so DPI > PPI (so a 2400DPI inkjet printer may not be able to do better than 600PPI printing)