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. Ufmylation of TRIP4 regulates nuclear receptors-mediated transcription. Interacts with UBA5. Interacts with UFC1. Belongs to the UFM1 family.