Below is an excerpt from the IRS Instructions (emphasis and bullets added).
There are exceptions to the rule that you must report each of your
transactions on a separate row of Part I or Part II.
Any taxpayer who qualifies can use Exception 1 or Exception 2.
Instead of reporting each of your transactions
on a separate row of
Part I or II, you can
report them on an attached statement
all the same information as Parts I and II and
in a similar format,
description of property
dates of acquisition and disposition
gain or (loss)
Use as many attached statements as you need.
Enter the combined totals from all your attached statements on Parts I and II with the appropriate box
For example, report on Part I with box B checked all short-term gains and losses
from transactions your broker reported to you on a statement showing basis wasn't reported to the IRS.
Enter the name of the broker followed by the words "see attached statement" in column (a).
Leave columns (b) and (c) blank.
Enter "M" in column (f).
If other codes also apply, enter all of them in column (f).
Enter the totals that apply in columns (d), (e), (g), and (h).
If you have statements from more than one broker, report the totals from each broker on a separate row.
Don't enter "Available upon request" and
summary totals in lieu of reporting the details of each transaction on Part I or II or attached statements.