The Sling Resource Merger provides services to access and merge resources. It provides diff (differencing) mechanisms for both:
Overlays of resources using configured search paths
Overrides of component dialogs for the touch-optimized UI (cq:dialog), using the resource type hierarchy (by means of the property sling:resourceSuperType).
Note: sling:resourceMerger concept is basically used for Granite. So this concept is specifically applicable for touch UI.
1. Approach for Overlaying Resources: Resource Overlay works on the path which is having the same hierarchy in /apps. If there is a path in libs: “/libs/text/example” then Its overlaying path can only be: “/apps/text/example”. This approach saves you from copying the entire structure of libs in apps when you need customizations.You can customize as much as required. 
Fig - Overlaying the resource
Example: Customization the consoles
Reference: https://sgaem.blogspot.in/2017/03/display-name-instead-of-page-title-in.html 
2. Approach for Overriding Resources: This approach works on the property sling:resourceSuperType. This concept is also called the inheritance of the resource in AEM.
Example: Configuring your page properties
Fig - Overriding the resource
Fig - How Override works in Resource Merger?
Goals of Resource Merger concept in AEM
1. Reduce the structure that is replicated from AEM and reusability of resources.
2. Ensure that any changes should not make in /libs.
Properties of Resource Merger
sling:hideProperties
String or String[]
Hide the properties, The wildcard(*)hides all.
sling:hideResource
Boolean
Indicates that the resource should be completely hidden with its children
sling:hideChildren
String or String[]
Hide the list of children of a particular resource. The wildcard(*)hides all the children.
sling:orderBefore
String
It contains the name of the preceding sibling.
Points to Remember
1. Overrides are not dependent on search paths. They use sling:resourceSuperType property to make a connection.
2. You must not change anything in /libs, The reason may be that when you upgrade your instance or apply any service pack/hotfix, It may be overwritten. So Any customization you needed should be done in /apps.
Use cases of sling: ResourceMerger:
Add a property
Redefine a property(not auto-created properties)
Redefined an auto-created property
Redefine a node and its children
Hide a property
Hide a node and its children
Hide Children of a node (while keeping the properties of the node)
Reorder nodes
Invoking the Sling Resource Merger from your code
The Sling Resource Merger includes two custom resource providers - one for overlays and another for overrides. Each of these can be invoked within your code by using a mount point:
Overlay:
purpose: merge resources based on their search path
mount point: /mnt/overlay
Usage: mount point + relative path
Example: getResource('/mnt/overlay/' + '<relative-path-to-resource>');
Override:
purpose: merge resources based on their supertype
mount point: /mnt/override
Usage: mount point + absolute path
Example: getResource('/mnt/override' + '<absolute-path-to-resource>');
Since Long, I am trying to find a good example of explaining /mnt/overlay and /mnt/override concept in resource Merger. So finally I got a use case to justify this concept.
Use Case: Recently I have gone through a scenario, where I want to add some more locales in the drop-down of page properties.
To know from where languages are getting listed out, I check the language widget in the page properties dialog.
So to populate languages, cq/gui/components/common/datasources/languages is playing an important role.
If I check the language.jsp under /libs/cq/gui/components/common/datasources/languages, you can see that all the languages are coming from /libs/wcm/core/resources/languages.
Now if I add some more languages in /apps/wcm/core/resources/languages, then the jsp will start taking the values from /apps not /libs, because /apps are having preference over /libs.
But I don’t want to duplicate all the locales in apps as well. So what to do?
1. Overlaying the hierarchy of the language: Just add all the new locales in /apps/wcm/core/resources/languages and now overlay the language.jsp with the following modifications.
2. Overriding the hierarchy of the language: Add new locales in any hierarchy example I have taken it “/apps/languages/core/resources/languages” and add a property sling:resourceSuperType to /libs/wcm/core/resources/languages. 
Overlay the language.jsp with the following modifications. 
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