Are you talking about the 8949 that sums up your investment sales?
If you're e-filing, you only need to mail that in if 1) you entered a summary for 1+ sales categories (instead of entering each individual sale) AND 2) one or more sales categories is something other than Box A (short-term covered) or Box D (long-term covered) OR your Box A or D summaries include adjustments from boxes 1f or 1g on your 1099-B.
This FAQ explains in more detail:
https://ttlc.intuit.com/questions/4080360-do-i-need-to-fill-out-or-mail-in-form-8949