CDLC packs are the single best way to turn Rocksmith 2014 into an infinite learning tool, provided you stick to trusted sources (CustomsForge) and avoid shady repack sites.
High-profile packs often come from "Charter" legends who ensure the note detection is frame-perfect and the tones are spot-on. Pro-Tips for CDLC Users Dynamic Difficulty: rocksmith cdlc pack
Before using custom songs, your game must be "patched" to recognize unofficial files. The DLL Patch : Download the D3DX9_42.dll CustomsForge Unlocking Infinite Tracks: The Ultimate Guide to Rocksmith