P is the number of pixels in the video quality. It goes from 144p,240p,360p,480p,600p,720p,1080p,4000p, and maybe more by now. I have not seen very many videos that have 600p though. I don't know anymore on how to answer this question. :P
Mostly because of viewers that live in rural areas. In urban and suburban areas most Internet is given by cables in the ground that give internet. Companies Do that because cables can reach several buildings or houses with families and make a lot of money. In rural areas it is less dense and not every house is right next to each other. So if the cables reach only 1 or 2 houses they prefer to not give cables because it doesn’t give companies enough money to balance the expense of making the cables. So most companies do satellites for internet in rural areas which currently isn’t the best. So rural viewers sometimes use 144p if the internet isn’t working.
Elaine Moss has written: 'Polar' -- subject(s): Children's stories 'Children's Books of the Year, 1979. Selected and Annotated by E. Moss (144p)' 'Peter Piper Books' 'The Peter Pipers in the Garden' 'The Peter Pipers in the garden' 'Reading for enjoyment with 2 to 5 year olds' -- subject(s): Books and reading, Bibliography, Children's literature, Children 'A sense of community' 'Paperbacks for children - two to eleven' -- subject(s): Bibliography, Books and reading, Children, Children's literature, Paperbacks 'The Peter Pipers at the Wildlife Park' 'Picture books for young people 9-13' -- subject(s): Bibliography, Books and reading, Children, Children's literature, Picture books for children 'Children's books of the year, 1970'