Hi all!

I'm getting used to the ropes in Krita, and I'm enjoying it so far. Now I'm trying to resize an image to 300 dpi because I'm printing some t-shirts for my design community club.
However, there's the option to "Adjust Print Size Separately." Excuse me if this is a dumb question. What does this do in Krita, what would be an example use case, and should I be checking this option if I'm intending to make t-shirts?

Unfortunately, after searching online, watching videos, and reading through the documentation, I haven't found an explanation. Hopefully, someone can help me here.

Thanks in advance!

The print size and dpi are only of interest if you're going to print, obviously.
Krita has the Measure Tool which uses the dpi value to determine physical distance across the canvas.
When you Export a .jpg or a .png, they contain the print size and dpi value inside them.

When you Scale an image, you have the option to separately adjust its printed size or dpi which is effectively the same adjustment.
I assume this can be used by automatic printing machinery to print an image exactly as stated in the dpi figure.

I'm sure that printing company personel are quite used to all kinds of strange file contents and mistakes with dpi values so for a t-shirt you'd tell them how big you want the image to be and they'd take care of the rest of the process. At least I hope they would.

You don't want to resize an image to 300dpi. You want to resize an image to whatever pixel size is needed to print the image at the physical size you want at a printed resolution of 300 dpi.
Ideally, you'd start the other way around and initially create an image that was the correct pixel size for the physical printed size you want at 300dpi.
It would be a good idea to ask for advice from the printing company.

As a matter of interest, I'd be surprised if it was possible to print on a t-shirt with an accuracy of 300 dpi.