Why do some clubs have square corner flags and other clubs have triangular flags?

Answer 1

Those who have triangular flags have won the FA cup. any one can use any shape of flag as long as you have a flag on top of corner pole - many clubs use Pennant style or Moon Flags to make them stand out you do not need to have won a cup or league competition

Answer 2

Only teams that have won the FA cup can have square corner flags! Any team that hasn't uses triangular flags.

Answer 3

It means that they have won the FA Cup in their history.