Per IRS Partner's Instructions for Schedule K-1 (Form 1065), page 1:
The partnership uses Schedule K-1 to report your share of the partnership's income, deductions, credits, etc. Keep it for your records. Do not file it with your tax return unless you are specifically required to do so. (See the instructions for Code O. Backup withholding, later.) The partnership files a copy of Schedule K-1 (Form 1065) with the IRS.
For your protection, Schedule K-1 may show only the last four digits of your identifying number (social security number (SSN), etc.). However, the partnership has reported your complete identifying number to the IRS.
Although the partnership generally isn't subject to income tax, you may be liable for tax on your share of the partnership income, whether or not distributed. Include your share on your tax return if a return is required. Use these instructions to help you report the items shown on Schedule K-1 on your tax return.
The amount of loss and deduction you may claim on your tax return may be less than the amount reported on Schedule K-1. It is the partner's responsibility to consider and apply any applicable limitations. See Limitations on Losses, Deductions, and Credits, later, for more information.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.