>[..] arguments in the standard input are separated by unquoted <blank> characters [..]
As for -i, it is documented to be the same as -I, which, among other things, makes it so that "unquoted blanks do not terminate input items; instead the separator is the newline character."
touch "a b" ls | xargs rm # this won't work, rm gets two parameters ls | xargs -i,, rm ,, # this will work