The National Archives has majority of the originals that were utilized in the early 1900's that were copied into the cards. Much of it is not well sorted, and catagorized by subject matter.

Regimental Records that the Federal government did not have in their possession or had already been "captured".... that the respective states still had were borrowed for the purpose of copying for the cards... then were returned to the respective states. If something of interest may be from a southern locale the archives of the state in question should have posession of the original documents. Most of the states have micro-fiche and/or film rolls of the copied originals that one can arrange to go sift through. It can get very time consuming trying to locate even simple items of ones research or interest.

Commonly when one requests a persons records they will give a couple items that would give confirmation of a persons service, which is all that most people want. If the person may have 20 documents associated with their respective record you might actually get only 5 of them. This is a common question when one gets the "records" and large gaps seem present. If you add in "Send All Copies" they will usually copy all they have to send you, but expect to pay whatever extra copying charge they may have.

Hope this helps
Frederick