Where both source and target are conceptual meanings, they cannot be linked as such by direct equality of color, size, or sound. We may observe signs associated with sleeping and death, yet the likeness between the meanings is an intelligible relation.
That is why the common feature in this category is conceptual. Selecting an outward effect as the proposed common feature requires justification that it truly belongs to both meanings in the same relevant respect.
The distinction preserves the difference between a perceptible effect and a conceptual meaning. Otherwise every abstract state with visible consequences would be wrongly reclassified as a sensory object.