Not every authentic report has identical strength. Even within sahih li-dhatihi, the precision of narrators, strength of continuity, number of routes, and external indications can produce different ranks.
In the ranking by source, the two Sahih collections' general reception and selection criteria provide significant presumptive weight. Accordingly reports found in the Sahihayn, reports meeting their criteria elsewhere, and sound reports in other collections are arranged into classes.
These are degrees of relative preference, not a declaration that a report belonging to a lower authentic class is therefore unusable. Authenticity itself and the comparative superiority of authentic reports must be distinguished.