How mappings works

SELECT DISTINCT ?s1 ?s2 ?g ?source ?o
WHERE {
 {  
     GRAPH <http://data.bioontology.org/ontologies/E-PHY/submissions/1> {     
      ?s1 <http://bioportal.bioontology.org/ontologies/umls/cui> ?o .
     } 
     GRAPH ?g {      
       ?s2 <http://bioportal.bioontology.org/ontologies/umls/cui> ?o .  
     }  
     BIND ('CUI' AS ?source) 
 } 
 UNION 
 { 
     GRAPH <http://data.bioontology.org/ontologies/E-PHY/submissions/1> {
      ?s1 <http://data.bioontology.org/metadata/def/mappingSameURI> ?o .
     } 
     GRAPH ?g {
      ?s2 <http://data.bioontology.org/metadata/def/mappingSameURI> ?o .
     } 
     BIND ('SAME_URI' AS ?source 
} 
UNION
{  
    GRAPH <http://data.bioontology.org/ontologies/E-PHY/submissions/1> { 
     ?s1 <http://data.bioontology.org/metadata/def/mappingLoom> ?o .
    } 
    GRAPH ?g {
     ?s2 <http://data.bioontology.org/metadata/def/mappingLoom> ?o .
    }
    BIND ('LOOM' AS ?source)
 } 
 UNION 
 { 
    GRAPH <http://data.bioontology.org/ontologies/E-PHY/submissions/1> { 
     ?s1 <http://data.bioontology.org/metadata/def/mappingRest> ?o . 
	  } 
    GRAPH ?g {
     ?s2 <http://data.bioontology.org/metadata/def/mappingRest> ?o .  
    }  
    BIND ('REST' AS ?source)}
 }
 FILTER ((?s1 != ?s2) || (?source = 'SAME_URI'))
 FILTER (!STRSTARTS(str(?g),'http://data.bioontology.org/ontologies/E-PHY'))
} 
OFFSET 0 LIMIT 50