MX Player Codec Pack not only contains ffmpeg library, It contains many other libraries required to work properly. If you download MX Player from Google Play mostly it will contain the required libraries according to your device architecture. You have to install codec packs from Google Play only if MX Player asks to install. Custom codec is just a modified version of the ffmpeg library used in MX Player. It's modified to include support DTS/Dolby(ac3) audio tracks. So, Codec Pack is mandatory if MX Player asks to install it. Custom Codec is optional and It's required only if you want to play the videos with DTS & Dolby (AC3) audio tacks