Elasticsearch

By the definition, SHOULD is like
an OR and without a MUST clause it implies 'at least' one should match.

must means: Clauses that must match for the document to be included. 

should means: If these clauses match, they increase the _score; otherwise, 
they have no effect. They are simply used to refine the relevance score for each document. 
Yes you can use multiple filters inside must
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