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DVD+R DL is the dual layer (see DVD-9 for more detailed info about dual layer) version of DVD+R format.
DVD+R DL's main difference to the DVD+R is that it can store upto 8.5 gigabytes on one disc side, unlike DVD+R (and other single layer discs, like DVD-R) which can only hold the same amount of data that a single layer disc can, appx. 4.38 gigabytes.
This means basically that every single one-sided movie disc ever released can be stored to one DVD+R DL disc without compression or removing anything from the original disc.