Abstract A specialization semilattice is a semilattice together with a coarser preorder satisfying a compatibility condition. We show that the category of specialization semilattices is isomorphic to the category of semilattices with a congruence, hence equivalent to the category of semilattice epimorphisms.
Guided by the above example, we recall an ``internal'' characterization of surjective homomorphisms between general relational systems. 
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