A "repack" is a version of the game where the files have been heavily compressed to reduce the download size. While the original FIFA 18 can exceed 40GB or 50GB (depending on DLC), a high-compression repack can reduce this significantly. However, unlike the original version, a repack must be "installed" or decompressed on your PC before playing, which requires a strong CPU and can take several hours.
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Once installed, the game files are typically identical to the original "lossless" retail version. FIFA 18 System Requirements A "repack" is a version of the game