When you search in multi-value fields there are additional options available than for Search in fields. To search in multi-value fields your properties must have Multiple values enabled, for more see Create a property section in Alfresco Content Services .
The following example queries are executed using a sample multi-valued property "mul:os" that stores values "MacOS" and "Linux".
mul:os:"MacOS"
Returns the document because "MacOS" is one of the values of the property.
mul:os:("MacOS" AND "Windows")
Does not return a document because the property doesn’t contain the value "Windows".
mul:os:("MacOS" OR "Windows")
Returns the document because "MacOS" is one of the values of the property, even though "Windows" is not.