In Player's Unknown Battle Grounds when playing single player there is an option for solo, and then another option for 1 man squad. What is the difference in these two options?
Solo is everyone playing by themselves.
In Squad, you get matched up with other players until you have a team of 4.
N-man Squad is the same as Squad, but you don't wait for other teammates. This means your team is at a disadvantage because it has fewer than 4 players, but it also means you get into a match quicker. So in particular, 1-man Squad means you'll be by yourself playing against teams of four.