The case-db node consists of nodes of cases. So when you use the statement instance('casedb')/casedb/case , you are telling CommCare to refer to all nodes of case-db. When you use instance('casedb')/casedb/case[@case_type = 'mother'][@status = 'open'], you are telling CommCare to refer to all the nodes of case-db whose case type is 'mother' and are still 'open' . Assuming that user has multiple cases of type 'mother' which are open, the output of this statement, instance('casedb')/casedb/case[@case_type = 'mother'][@status = 'open'], is always a set of nodes of cases (or this will result in Multiple Nodes)

Also, when you use instance('casedb')/casedb/case[@case_type = 'mother'][@status = 'open']/case_name, this will result in multiple nodes with just case_name

When you face the issue "Cannot convert multiple nodes to a raw value. Refine path expression to match only one node." in a repeat group, it means that in one of your question's calculations (or any configuration of the question) is expecting a single node but as per the logical statement you are using results in multiple nodes and CommCare is unable to resolve as to which node it should use do process your logical statement. For example, consider an application which registers a mother (case type : mother) and her children (case type : children) . Assuming that you have registered more than 2 mother cases and each mother case has 3 children cases created with a relation of parent-child between these two case types. There is a form which will now show the user the name of mother along with the list of children under each mother. For this, form uses a repeat group to iterate through the 'mother' case type nodes.

Now let's say you wanted to show the details first, before showing the details of the children of her children. For that you added a hidden variable type question to fetch the name of the mother whose case-id is currently in the loop. In that hidden variable's calculation, you added this calculation instance('casedb')/casedb/case[@case_type = 'mother'][@status = 'open']/case_name
Now , when you try to open the form, you will get the error "Cannot convert multiple nodes to a raw value. Refine path expression to match only one node" and the reason is clear that the calculation used for the hidden variable is resolving to a set of nodes of cases and not a single case as shown :