.7Z.001 File Extension

Part 1 of a compressed archive created by 7-Zip, a free file compression and decompression utility; contains one part of a multi-volume archive that splits a .7Z file into parts (other parts use the ".7z.002," ".7z.003," etc. extensions); enables large 7Z files to be broken into smaller parts for piecewise download or for storage on external media with fixed capacities.

You need all all parts of a 7-Zip split archive in order for 7-Zip to rebuild the original ".7z" file. Once 7-Zip has reconstructed the 7Z file, it can then decompress the 7Z file to extract the files from the compressed archive.

NOTE: 7Z.001 files use a compound extension, which uses both .7Z and .001 extensions.