Here, “disord.” means a disordered (flexible) loop: the tryptophan (W) and adjacent residues are not seen in the experimental 3D structure determination 6VVX. This is also true of PDB entry 6VSB the spike protein in the open state, 6VYB and other SARS-CoV-2 accessible to the author at the time of writing. See Fig. 3 . The fact that disordered suggest an open loop like structure that be may be capable of binding to, and perhaps adjusting to, disordered targets, and certainly does not prohibit functional significance.
Fig. 3 S1-NTD (PDB 6VXX) Showing Residues at Boundaries of Invisible (Disordered) Segments and the Location of the Two Visible Tryptophan (TRP) Residues. These are the last visible residues of the non-disordered region bounding a disordered region, and this figure shows how the missing sections result in “cut ends” in displays of the reported three dimensional structure.