The Invalid Corps
The corps was organized under authority of General Order No. 105, U.S. War Department, dated April 28, 1863. A similar corps had existed in Revolutionary times. The Invalid Corps of the Civil War period was created to make suitable use in a military or semi-military capacity of soldiers who had been rendered unfit for active field service on account of wounds or disease contracted in line of duty, but who were still fit for garrison or other light duty, and were, in the opinion of their commanding officers, meritorious and deserving.
The Veteran Reserve Corps
The title "Veteran Reserve Corps" was substituted for that of "Invalid Corps" by General Order No. 111, dated March 18, 1864. The men serving in the Veteran Reserve Corps were organized into two battalions; the First Battalion including those whose disabilities were comparatively slight and who were still able to handle a musket and do some marching, also to perform guard or provost duty. The Second Battalion was made up of men whose disabilities were more serious, who had perhaps lost limbs or suffered some other grave injury. These latter were commonly employed as cooks, orderlies, nurses, or guards in public buildings.
Hope this helps you to understand my using both titles