Seasonality can have a big impact on the results you expect in any given period. The easiest way to add seasonality into your plan is to create an initiative to specifically model the seasonality you expect.
If you want to model in seasonality into your plan goals, then creating an assumption is the best way to achieve this.
Remember, the impact of initiatives and assumptions are applied in a specific order when calculating your projection and plan goals.
Talk to your Growblocks CSM if you need advice on how to add in seasonality specifically for your business.