What kind of finish is the 'seam allowance binding' mentioned in the description?

A seam allowance is the extra fabric left outside the stitching line when sewing two pieces of cloth together. When you turn a garment inside out, that edge of the fabric standing inside the seam is the seam allowance, and if left as is, the threads will fray. Binding refers to the method of encasing and stitching over that edge with a narrow strip of fabric.

This jacket is unlined on the inside, and the seam allowances are finished by wrapping them with binding instead^[1]. In a lined jacket, seam allowances are hidden behind the lining, so there is no particular need to wrap them. When the lining is removed, that area is exposed as is, so how the seam edges are finished becomes a finish directly visible on the inside of the garment.

How an unlined garment actually wears depends on the nature of the outer fabric. In general, when there is no lining, the weight of the garment becomes lighter and the drape of the outer fabric shows through directly, but slipping against the body or inner clothing is reduced, giving a different feel when sliding your arms in and out. For light colors, it is also better to check how much it shows through in light. However, what the outer fabric of this jacket is and why it was left unlined were not confirmed in official materials.

hmmm, the single line stating there is no lining is closer to a statement disclosing how the inside was finished as a result, rather than merely indicating what was omitted.