ZIP
The ZIP file format is a standard container format developed by 
PKWARE and allowing a large number of compression methods. This is an open and widely used standard; however it is very seldom implemented fully.
The standard defines the following compression methods (bolded methods are supported by 3DGE):
0 - The file is stored (no compression) 1 - The file is Shrunk 2 - The file is Reduced with compression factor 1 3 - The file is Reduced with compression factor 2 4 - The file is Reduced with compression factor 3 5 - The file is Reduced with compression factor 4 6 - The file is Imploded 8 - The file is Deflated 9 - Enhanced Deflating using Deflate64(tm) 10 - PKWARE Data Compression Library Imploding (old IBM TERSE) 12 - File is compressed using BZIP2 algorithm 14 - LZMA (EFS) 18 - File is compressed using IBM TERSE (new) 19 - IBM LZ77 z Architecture (PFS) 97 - WavPack compressed data 98 - PPMd version I, Rev 1 (as of Template:Latest, only PPMd version H is supported (.7z/.pk7, not .zip/.pk3))
Each file in a ZIP archive can be compressed with a different method. In practice, ZIP implementation is generally limited to only formats 0 (stored) and 8 (deflated).
3DGE and zipdir both support methods 0 (stored), 1 (shrunk), 6 (imploded), 8 (deflated), 12 (bzipped 2) and 14 (LZMA, also used by the 7z format). As of 2009, when Template:Forum, all the zip files on the Idgames archive can be opened successfully by 3DGE (unsupported methods are not used).
The implementation in 3DGE is supported via the use of Physics_FS, by Ryan C. Gordon.