The resolution indicates how many color dots occur on a given length. The unit of measure dpi stands for "dots per inch" and ppi for "pixels per inch". From pixels will be spoken mostly on monitors and displays and from dots when material color dots incur, for example when printing. A color point in turn can be composed of several individual colors. In this way, an RGB display uses the three colors red, green and blue for displaying one color point. One inch equals 2.54 cm in metric system. So, both units of measure state, how many color points occur on 2.54 cm. As an example: With an resolution of 150dpi, there are 150 color dots being printed
over a length of one inch. This equals 5.9 points per mm.
In the Stock Photography the photos are first created and then sold or licenced. The Stock Photography is thus the contrast to the commercial photography, where the customer is known befor. Stock photos "live" by the expectation to be used and sold some day, since the capital expenditure rests with the artist or an agency first. The aim of Stock Photography is thus to image today what will be needed tomorrow.