When a male, typically a canine but not restricted to them, has a full erection but their sheath is not fully retracted. This is most commonly seen in canines as the knot, once it swells, will become too large to exit the sheath. For an image of a canine to qualify for this tag, there must be a visible knot-bulge! A canine's penis is not fully erect until the knot has started to swell. Most other species may only qualify for this tag when the sheath is being physically held in place over the erect penis by a hand or other means, as most will lack a knot to do so from inside.
Remember, knot-bulge or externally held in place over the erect penis, otherwise a full erection is likely not present.