If an org hits the account product period forecast record limit, it means that the org has reached the maximum number of records allowed for the Account Product Period Forecast object, which is 9 million1. This limit affects the functionality of account forecasting in the following ways:

New products are not added when recalculating a single account forecast or recalculating all account forecasts. This means that if a new product is added to a sales agreement or a contract after the limit is reached, it will not be reflected in the account forecast, even if the forecast is recalculated manually or automatically. The only way to add new products to the account forecast is to delete some existing records from the Account Product Period Forecast object to free up some space2.

The add products option will no longer appear on the agreement terms tab. This means that users will not be able to add new products to an existing sales agreement or a contract from the agreement terms tab. The only way to add new products to an agreement is to create a new agreement with the new products, or to edit the agreement in the related list of the account forecast2.