It’s an annual ritual for some investors: watching others file their tax return and pocket their refund while you wait, and wait, for the Schedule K-1 tax forms needed to complete your taxes.
Partnerships and other entities that pass their tax liability on to shareholders must send investors K-1 forms by March 15, or if they don’t operate on a calendar year, by the 15th day of the third month following the date their tax year ended. The form shows investors’ share of income, losses, and dividends from the business. Partnerships include master limited partnerships and commodity ETFs that operate as limited partnerships.
Source: https://www.barrons.com/articles/taxes-k-1-partnerships-k-3-4757e645?siteid=yhoof2&yptr=yahoo