Generic Resources are usually used for planning purposes when you don't know for sure which resource can handle your task at a certain date. You have the possibility to assign a Skill set (AD, SQL, Networking) to this generic Resources and to assign this skill set to a real Resource as well. This way you can exchange at a later time the Generic Resource with a real Resource that corresponds to that specific skill set.
You can achieve this as follows:
Under Server settings - Create a Lookup Table with the needed skill set of your resources and save it.
Create a Resource type Enterprise custom field that is using that Skillset Lookup table:
Make sure to check the "USE this field for matching generic resources" and "allow multiple values to be selected from lookup table" checkbox
Then Create several Generic Skill Resources (Server settings - Resource center) for every skill set that you created in the lookup table for your resources (like AD, Exchange, Project, SharePoint in my example) and assign under the resource custom field the specific skill.
Afterwards assign the specific skill or multiple skills for the resources.
In the Projects you can afterwards from the Build Team from Enterprise option match the resources with the specific skill set that you are needing for the generic resource or resource.
Just select the Generic Resource from the right and click the match button and you will see the specific resource that has the required skill set and you can replace it afterwards.