Pack-n-play sheets are for those little crib things that are used as cribs for infants and turn into playpens for little older babies.
Since my youngest baby is 54 yrs old, I never used any of these new contraptions. The crisis pregnancy center I sew for gave me a sample sheet to copy and told me that they are in high demand. I have been making them for several years now and donate about 100 or so every year. They are very easy to make.