Ubiquitin-like modifier which can be covalently attached via an isopeptide bond to lysine residues of substrate proteins as a monomer or a lysine-linked polymer. The so-called ufmylation, requires the ufm1-activating E1 enzyme uba5, the ufm1-conjugating E2 enzyme ufc1, and the ufm1-ligase E3 enzyme ufl1. Ufmylation is involved in reticulophagy (also called ER-phagy) induced in response to endoplasmic reticulum stress. Interacts with uba5. Interacts with ufc1. Belongs to the UFM1 family.