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A DAT file (having the extension .dat) is the file format which Chip's Challenge level sets are commonly stored in.
Since the release of CCTools, level sets are sometimes stored in CCL files (having the extension .ccl, which stands for Chip's Challenge level set). For people who have ChipEdit, which does not provide a way to open CCL files "normally", simply change the extension back to .dat, or type the file name manually when in the Open dialog.