By default, Salesforce will forecast using Amount field from Opportunity for Revenue forecast. However, to fit your business needs, you can add additional forecast type using custom fields in Opportunity, and the fields must be a currency field.
1. Opportunity Split
In this blog, I will add a custom currency field from Opportunity called Extra Income. I'll select the field 'Extra Income' and type in 'Extra' as Split Label.
2. Forecast Setting
Now, I need to configure forecast setting, click the link "+ Add another forecast type" then select Extra.
Then, you select a Forecast Measurement (Revenue or Quantity) and select fields to show in the Opportunity List.
You need to click Save button to save, otherwise, it will not save, even you have click OK button in many screens.
Now, let us see if this will work.
This is the default forecast based on Amount, it called Opportunity Revenue.
Now, let us flip to the new Extra forecast we just create, click the gear icon on the top right.
Now, let us forecast with our new forecast type "Extra"
Reference: Enable Custom Field Forecasts in Collaborative Forecasts