Q. Why can't you use

High/Print Resolution is essentially the physical printable size of a digital file format such as a bitmap jpg at a value of 300dpi or more correctly a PPI (pixels per inch) value. This number lets the designer/printer know the intended print size of the image.

Digital Photography is usually stored as a rasterised graphic (made up of pixels) and is resolution dependent. They cannot scale up to an arbitrary resolution without loss of apparent quality.

 

For example:

If you provided a photograph as a bitmap jpg and it measures 100x100mm when labelled as 300ppi then that will be it's maximum physical printable size at "High/Print Resolution" on a conventional printing press.

However if you wanted to physically print the image at a larger size, say 200x200mm, which is a 200% increase in size, it would actually reduce the true PPI to 150ppi, making the image appear blocky and out of focus, blurred, this is called "Pixelation". "Pixelation" is a problem unique to bitmap or raster graphic images, which are data structures representing a generally rectangular grid of pixels, or points of colour. This is why this loss of quality occurs as it just increases the actual size of each pixel which in turn makes them more visible to the naked eye.

Now, you could resample this image within an image manipulation package like Adobe Photoshop, by keeping the ppi at 300 and increasing the dimensions to 200x200mm, but the same "Pixelation" issues would occur when printed on a conventional printing press.

However if you make the original image smaller instead, say 50x50mm, this would increase the actual ppi to 600ppi, no visible effect would take place as the press screen resolution is unable to resolve this extra detail.

Most modern computer-monitors typically display about 72 to 130ppi called "Screen Resolution". Generally, the industry standard is to produce imagery at 72ppi for websites, which when extrapolated up to "High/Print Resolution" 300ppi come out at a very small physical print size, often far too small to be used without being very pixelated.