Confusion between the meaning of seasonality and seasonal patterns in time-series forecasting

According to Forecasting: Principles and Practice Seasonality is always of a fixed and known frequency. If it is a fixed and known frequency, does that mean every series with monthly or quarterly data can use methods that captures seasonality? I am asking because I am not sure if I could use SARIMA and Holt-Winters with my monthly data because there seems to be no seasonal pattern based on my ACF plot. But I've seen some papers have D=0 in their SARIMA model.