what is meant by seasonality mode in prophet

1# R
2df <- read.csv('../examples/example_air_passengers.csv')
3m <- prophet(df)
4future <- make_future_dataframe(m, 50, freq = 'm')
5forecast <- predict(m, future)
6plot(m, forecast)
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8# Python
9df = pd.read_csv('../examples/example_air_passengers.csv')
10m = Prophet()
11m.fit(df)
12future = m.make_future_dataframe(50, freq='MS')
13forecast = m.predict(future)
14fig = m.plot(forecast)
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