How Source Date of the Rev Rec Schedule is determined?

• Source Date – The date of the source customer performance obligation that generates the revenue element, for example, the date of a sales order. Forecast revenue recognition plans use the source date as the event date.

I found the description related to my question, but too rough to understand.

What is "The date of the source customer performance obligation"?

How Source Date of the Rev Rec Schedule is determined?

Can users indicate which date field should be Source Date on each transaction type?

If it's determined by the specification, I would like to get the list of the relation between the Source Date and each transaction type.