Schedule D-1 is a California schedule used to report the sale of business property.
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Schedule D-1 is the first "continuation" page of capital gains/losses transactions. Capital gains and losses are considered owned by the taxpayer, not owned by a business. So they are not business assets, but instead, personal assets sold.
Once the available lines on the actual Schedule D of the return is full, it then carries the remainder to the Schedule D-1 Continuation Sheet.
Generally, if the cost basis of all transactions are considered "reported" to IRS, then the 8949 does not have to be completed and the transactions just go to the Sch D.
If the 8949 is used, then the transactions are also listed by holding periods and category on Form 8949 and then carry to Schedule D and any remaining carry to the D-1.
[Edited 02/21/2021|5:44 pm PST]