All Types of Price Reductions
Amazon's API considers sales with all kinds of price reductions as promotions. Shipping price or cross-product reductions will also fall into this category.
If a product was bought jointly with other (competitor) reduced products, it will sometimes also be transmitted to Sellics as a Promo. The same goes for certain global promotions by Amazon.
Reductions of Shipping Revenue transmitted as Promo Fees
As described in further detail here, FBA sellers receiving "shipping revenue", for example from non Prime Customers, will have the same amount deducted within other fees. Amazon's API will include this amount within the "Promo" fees.